What is the cutoff between repairing and replacing roof for flip?

[Nam Hoang]

I'm just starting out with RE, so forgive me if this question is overly basic, but I could not find a straight answer in the search.  When looking at houses and figuring out whether or not to repair or replace the roof, how exactly do you decide?  If an asphalt roof has a life of 20-30 years, and your flip's roof is in decent condition after 15 years, would you simply do some roof maintenance?  Basically, what are the factors that I should consider to make this decision?