It depends on which ball you buy.
Two and a half Durability dots is the most durability. IIRC 2.5 dots is good for 110-115 hits.
On a per round basis, the fewer times you hit a ball the longer it will last.
Putts on the green don't count against Durability.
So if you can score 70 for a round and you made 30 putts, only 40 hits are deducted from the Durability of a ball. So at 115 hits for 2.5 dots, a ball will last about 3, 18 hole rounds.