I have a cheap shared hosting plan with Reliablesite.net ($5/month).
I've been making a small site I want to start promoting in a few weeks and I was going to roadtest it by hosting it with the shared plan I already have.
My issue is that I don't know at what point I should move onto clustered hosting / dedicated hosting.
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!