The impossible dream
Chad is correct. Asking how much EQ to use is like asking how much salt to use, it totally depends on what is on one's plate at the time and one's own taste.
That said, however, I'd like to humbly offer a few
general offerings on how EQ is often best used (there are always exceptions)...
1.) Try to use EQ in mixing to make stuff sound better, not to make it sound different. Shoot for getting the right sound in the tracking/recording phase, and use EQ only for any small necessary adjustments in the mixing phase. If you have to make major EQ adjustments in mixing, that is usually an indication that someting went wrong during tracking, and - if time and budget permit - might be best off being re-tracked.
2.) Use EQ boosts to make things sound different. Use EQ cuts to make things sound better. (This is a
very general rule, but it's a useful one to remember.)
3.) When you have a busy mix or complex arrangement, on miked instruments, depending on the instrument, use high or low shelving as appropriate to cut off unused or unnecessary frequencies for that instrument. This will "make more room" in the sonic spectrum for the other instruments and allow them to fit in the mix together better. For example on a kick drum you might shelve off the mid-high to hi frequencies whereas on an
acoustic guitar you might shelve off the low bass, etc.
4.) It often sounds better to boost the lower harmonic and higher harmonic of a particular frequency by a couple of dBs than to boost that particular frequency by several dBs. For example, if you feel you need a real boost at 700Hz, sometimes just slightly nudging up at 350Hz and 1400Hz will have a better sound than a super boost at 700. Sometimes. Try both individually and see what happens.
5.)Try to do as much EQing as possible on the individual tracks before you mix down to stereo. Keep EQing of the mixdown itself to minimal sweetening.
There are more tips and techniques for EQ, but they start straying from the thread. These hould hopefully give you some ideas at least.
HTH,
G.