It really depends. For alot of work your are only correcting photos, getting the exposure, contrast and colour right and so on. But as far as being creative thre is no limit really, its dont to your own creative freedom. I found as i learned new techniques i tended to over use it untill i found another technique i wanted to try and the cycle continued. But after a while you will start to develop an instinctive judgment as to whether a photo needs extra PP or not, and what it needs for it to work.
I'd say just keep at it and you will develop your own styles and techniques all in good time. I'd also add that, in most instances you'll want to have a set of pictures to have the same PP style. But you can switch from different styles from set to set, shoot to shoot etc. Its good for each shoot to have its own distinct style, you just gotta be careful about doing it for doing its sake.
I'd recommend taking a look at coffeshopblog and scouring youtube for cross-processing techniques, David Hill photoshop tutorials, dragan also. Just search the words 'photoshop' with other words like, glamour, fashion, portrait, retro etc
What kind of stuff do you shoot?
