Read the DATASHEET for your PIC... especially the ADC section.

Instruction Cycle is FOsc/4 so a 4MHz chip has 1uS Instruction Cycles.

It will probably take you 60uS to do ONE ADC (aquisition plus external housekeeping, retrieval, storage etc). Your worst case scenario is 1.4kHz... that means one complete sine-wave in 714uS... you'll be lucky to do six acquisitions per half wave... if you're lucky you'll hit peak, if you're unlucky your best shot will have missed peak by 15 Degrees. So doing my Nostradamus bit and predicting the future, you'll have a resultant randomly waving about like a pr**k in distress by upto 15 degrees of Sinus.

Oh... and you know you're talking about not having any (or minimal) external components... unless your input has a DC bias (more components needed), half your input will be negative going... so for 357uS out of the 714uS the PIC isn't going to see anything, but worse, it's internal protection circuitry on it's pins may well load and distort your input signal during the negative half-cycle, so unless the signal is buffered (more components), you'll distort the output of your Guitar by the simple act of connecting up.

But don't let me put a damper on things... it's your life...