How often or when should one replace valvesprings?

I know that in time they lose spring pressure, but when is the time to change them? How many miles till' it's time ? Thanks.

 

Depends on the size of the cam. I would check them every 10k miles or so.

 

There is no real XX,XXX miles and they are bad...
The only way to check them is with a spring tester, and verify that they have the appropriate open and closed spring pressures. If they do, then they are good, if not, they need replacing.
You could probably get away with checking them once a year or after 10-15k miles.