How do I run a loop for a specific amount of time?

Here is an example that will run for 5 minutes. Note that the loop will begin executing anytime before the time limit is up, including 1 msec before; it can't cut-off something happening at the 5-minute mark, meaning the timing precision will be limited to the duration of the code in the loop.

Update: My first suggestion had a bug related to the 50-some-odd days roll-over period of the millis() clock. This one is more robust:

uint32_t period = 5 * 60000L;       // 5 minutes

for( uint32_t tStart = millis();  (millis()-tStart) < period;  ){
   doStuff();
   moreStuff();
}

You can use a variable that is incremented each time you perform the loop, then you need only to change the loop from using a for to a while.

For example:

   int incremented_variable = 0;
   int incremented_value = 100;
   int max_value = 1000;  

   while(incremented_variable < max_value){
   if (kill() == true){ break; }

   strip.setPixelColor(1, strip.Color(random(100,255),random(100,255),random(100,255)));
   strip.show();

   tone(TONE, notes[random(0,3)]);

   delay(100);
   incremented_variable = incremented_variable + incremented_value;
   }

---

EDIT: I have just read your update on the question.

You can then use a while with a boolean flag that help you to end the loop or if you prefer, you can keep track of the time by calling the method: millis().

Using this example:

http://playground.arduino.cc/Code/ElapsedMillis

You could try something like:

#include <elapsedMillis.h>

elapsedMillis timeElapsed;
unsigned int interval = 60000; //one minute in ms

while(timeElapsed < interval){
    //do stuff
}

Depending on how accurate you want to be, you might want to opt for a similar method using the RTC.