I can't find any numbers, but I'd say they are fairly common. There are several things that can cause an abnormal pap: infection or an inflammation like yeast infections, herpes, trichomonas, recent sexual activity, or use of vaginal preparations. Often the reason for an abnormal pap is HPV. It is important that if you have an abnormal pap to be retested in a few months (3 or 4, I think is average) and/or ask for a colposcopy.
Actually, the main cause of an abnormal PAP is HPV. There can be other reasons - including inaccurate test results - but it is mainly HPV.
As for the question, I'm not quite sure what you're asking. Do you mean you have HPV, have had a few normal PAPs, but now you've had an abnormal one? Or you've had an abnormal one and are wondering how long you've had it?