How Much Traffic Can Shared Web Hosting Take?

In my experience, shared hosting environment like Reliablesite.com can take around 10-20 000 unique users per day, or 100-200 000 pageviews/day. That number can vary, depending on your site. For optimization, It is important to reduce number of db queries (i keep it max 6-7 per page render), and be careful when programming. Using ASP.NET MVC gave nice perf improvement for me, but good written webforms app can perform well also. If you are using some other tech stack, like PHP/MySQL, i don't know the numbers.
When you exceed those numbers, you will have enough money from google adsense to go with VPS or dedicated plan.

Just to add something regarding page render / db queries performance: using multiple resultset sproc or query is great way to reduce number of db requests!

It would be best if you ask your provider these questions. Every provider is going to be different.

Generally what happens is that the provider can handle the requests, but they'll simply shut down your site once it reaches a certain threshold.

Traffic usually is not a problem on shared hosting. The only problem you may encounter is RAM and CPU restrictions. But if your application written correctly it could operate well with these limitations.

Hints:

• user memory profiler to debug and optimize your web application
• use CDN for storing media files

If you need some numbers, a properly written web application which use CDN for storing media files could handle at least 10k unique visitors per day on a shared hosting.