Usually 10-15 minutes, not unusual for it to go to around 20 sometimes. I haven't seen past 25 I don't think, that would be both teams wiping each other every fight. The objectives just push the game to a conclusion pretty quickly, since they are very low risk to start and kill towers/buildings either directly or indirectly. It's sort of like if dragons were weaker, spawned every 3 minutes, and made a rift herald if you kill one.