How common is it for a case of genital warts once cleared to return years later? does this show up in practice or studies often?

Pretty uncommon: Genital warts can reappear several years after an initial episode. There are no statistics, but it's clearly uncommon; I would guess it happens in under 1% (and certainly less than 10%) of people with genital warts. Some probably are new HPV infections, but of course that depends on risk of new exposure to and infected partner. It's an inconvenience, not a serious health risk. Good luck!