How would I stop a while loop after n amount of time?

Try this module: http://pypi.python.org/pypi/interruptingcow/

from interruptingcow import timeout
try:
    with timeout(60*5, exception=RuntimeError):
        while True:
            test = 0
            if test == 5:
                break
            test = test - 1
except RuntimeError:
    pass

You do not need to use the while True: loop in this case. There is a much simpler way to use the time condition directly:

import time

# timeout variable can be omitted, if you use specific value in the while condition
timeout = 300   # [seconds]

timeout_start = time.time()

while time.time() < timeout_start + timeout:
    test = 0
    if test == 5:
        break
    test -= 1

Try the following:

import time
timeout = time.time() + 60*5   # 5 minutes from now
while True:
    test = 0
    if test == 5 or time.time() > timeout:
        break
    test = test - 1

You may also want to add a short sleep here so this loop is not hogging CPU (for example time.sleep(1) at the beginning or end of the loop body).

Petr Krampl's answer is the best in my opinion, but more needs to be said about the nature of loops and how to optimize the use of the system. Beginners who happen upon this thread may be further confused by the logical and algorithmic errors in the question and existing answers.

First, let's look at what your code does as you originally wrote it:

while True:
    test = 0
    if test == 5:
        break
    test = test - 1

If you say while True in a loop context, normally your intention is to stay in the loop forever. If that's not your intention, you should consider other options for the structure of the loop. Petr Krampl showed you a perfectly reasonable way to handle this that's much more clear to someone else who may read your code. In addition, it will be more clear to you several months later should you need to revisit your code to add or fix something. Well-written code is part of your documentation. There are usually multiple ways to do things, but that doesn't make all of the ways equally valid in all contexts. while true is a good example of this especially in this context.

Next, we will look at the algorithmic error in your original code. The very first thing you do in the loop is assign 0 to test. The very next thing you do is to check if the value of test is 5, which will never be the case unless you have multiple threads modifying the same memory location. Threading is not in scope for this discussion, but it's worth noting that the code could technically work, but even with multiple threads a lot would be missing, e.g. semaphores. Anyway, you will sit in this loop forever regardless of the fact that the sentinel is forcing an infinite loop.

The statement test = test - 1 is useless regardless of what it does because the variable is reset at the beginning of the next iteration of the loop. Even if you changed it to be test = 5, the loop would still be infinite because the value is reset each time. If you move the initialization statement outside the loop, then it will at least have a chance to exit. What you may have intended was something like this:

test = 0
while True:
    test = test - 1
    if test == 5:
        break

The order of the statements in the loop depends on the logic of your program. It will work in either order, though, which is the main point.

The next issue is the potential and probable logical error of starting at 0, continually subtracting 1, and then comparing with a positive number. Yes, there are occasions where this may actually be what you intend to do as long as you understand the implications, but this is most likely not what you intended. Newer versions of python will not wrap around when you reach the 'bottom' of the range of an integer like C and various other languages. It will let you continue to subtract 1 until you've filled the available memory on your system or at least what's allocated to your process. Look at the following script and the results:

test = 0

while True:
    test  -= 1

    if test % 100 == 0:
        print "Test = %d" % test

    if test == 5:
        print "Test = 5"
        break

which produces this:

Test = -100
Test = -200
Test = -300
Test = -400
...
Test = -21559000
Test = -21559100
Test = -21559200
Test = -21559300
...

The value of test will never be 5, so this loop will never exit.

To add to Petr Krampl's answer, here's a version that's probably closer to what you actually intended in addition to exiting the loop after a certain period of time:

import time

test = 0
timeout = 300   # [seconds]

timeout_start = time.time()

while time.time() < timeout_start + timeout:
    if test == 5:
        break
    test -= 1

It still won't break based on the value of test, but this is a perfectly valid loop with a reasonable initial condition. Further boundary checking could help you to avoid execution of a very long loop for no reason, e.g. check if the value of test is less than 5 upon loop entry, which would immediately break the loop.

---

One other thing should be mentioned that no other answer has addressed. Sometimes when you loop like this, you may not want to consume the CPU for the entire allotted time. For example, say you are checking the value of something that changes every second. If you don't introduce some kind of delay, you would use every available CPU cycle allotted to your process. That's fine if it's necessary, but good design will allow a lot of programs to run in parallel on your system without overburdening the available resources. A simple sleep statement will free up the vast majority of the CPU cycles allotted to your process so other programs can do work.

The following example isn't very useful, but it does demonstrate the concept. Let's say you want to print something every second. One way to do it would be like this:

import time

tCurrent = time.time()

while True:
    if time.time() >= tCurrent + 1:
        print "Time = %d" % time.time()
        tCurrent = time.time()

The output would be this:

Time = 1498226796
Time = 1498226797
Time = 1498226798
Time = 1498226799

And the process CPU usage would look like this:

That's a huge amount of CPU usage for doing basically no work. This code is much nicer to the rest of the system:

import time

tCurrent = time.time()

while True:
    time.sleep(0.25) # sleep for 250 milliseconds
    if time.time() >= tCurrent + 1:
        print "Time = %d" % time.time()
        tCurrent = time.time()

The output is the same:

Time = 1498226796
Time = 1498226797
Time = 1498226798
Time = 1498226799

and the CPU usage is way, way lower: