How common or rare is it to do a hypospadias repair surgery in an adult?

Not very common. Some parents, in my opinion incorrectly, decide to postpone repair until boy is a young adult. These boys are usually full of anger and wish they had been repaired as babies. Some adults have come from countries without hypospadias surgeons. Parents of some children are advised against any surgery because defect thought to be too minor, may request repair as adults. 5% of my patients are adults.
Commonly done. Hypospadias unattended into adulthood (depending on the severity) could result in problems with hygiene ("splaying"/spraying of urine stream, dribbling, .) or infertility (inability to deposit semen into vaginal vault). Repair is similar to that performed in a child and is usually as successful.