The answer to that question depends completely on the
robustness of your system. Nobody can give you
a flat rule that covers all systems. There are many factors
to consider:

1. System load - How many people / applications will
be running on your system.

2. How much memory does your system have?

3. Are you running *nix or winblows?

4. What are you dbms limitations?

5. What OS version are you using?

6. Is your app transaction-based or scheduled?

Answering those questions (and undoubtedly many more)
will give you an idea of where to start with your
testing.

Good luck.