Pressure testing

[syata]

How often should you pressure test your Rolex?

I swim quite often and haven been locking up my Rolex (SUB, GMT, Explorer II) in the locker because I don't want to risk having water leak into the watch case.

I figure I should have my watches pressure tested before I take them into the water just to be sure.

After they are pressure tested when would you have them pressure tested again?

Thanks.

Stephen