What is the point of having paxil (paroxetine) or zoloft to treat premature ejaculation if its side effect decreases libido? how many doses will effect the libido?

It may not occur: Those medications don't always cause low libido. A young otherwise healthy male with a regular libido (prior to starting meds) is probably not likely to get decreased libido with treatment, though it could happen. Work with your doctor to find the appropriate dosage and schedule for treating your premature ejaculation without decreasing sex drive. SSRIs can work very well for this problem.

Variable: Paxil (paroxetine) and zoloft can but don't always decrease libido. Not everyone experiences this side-effect which can happen with one SSRI and not another and can't be predicted. It can happen after the first dose or only after you have been on it for awhile. Unfortunately,there is no way to predict how either of these will effect a particular individual. Hopefully, personalized genetic medicine will help