As far as they have to. Intact (not neutered) tom's can smell a queen (female cat) in heat from over six miles when the wind is right. All it takes to travel indefinitely is another female in heat every few miles and no tougher tom marking the territory between. The only way to stop the roaming is to have your kitty neutered the next time he wanders home, assuming he feels like coming back at the end of mating season and hasn't found a colony of females to claim on a new territory. Every intact male cat I have ever encountered has wandered from home for at least a week and many never came back at all. There is an enormous number of unclaimed and feral cats in this world, so for everyone's best interest, always have your pet spayed or neutered by six months old.