At 30 minutes you get a last boss, if you kill it you have the option to exit or go endless. Generally if you reach the 30 minutes mark you are so over powered that it is better to restart a game (continuing can cause lag because your attack speed is too high for example).
The lifetime of the game (buying all characters/upgrades) is currently around 3 to 4 hours for new players imo. On the last update I "finished" the game in 2h approximatly). I did 4 runs to completly max everything.