This isn’t a simple question to answer because it depends on a player’s personal preferences and how much playing they do, but generally speaking if the hair seems slack, loses its natural bounce or grip with the string (even after putting on rosin) then this is a sure sign a rehair is needed.
As the bow hair stretches over time it displaces tension along the hair which causes uneveness. We recommend students get a rehair every 6 months to a year. Aside from the problems listed above our greasy hands tend to rub off on our bow hair (especially at the heel). Always ask your teacher to check over your bow if in any doubt!