2 Minutes.
Now yes, if you're literally breathing in water for 2 minutes you're probably dead. But even if you pass out at the minute mark you should be fine after another minute.
Lucas should have known that.
At my best I could get 1:35 but I never trained specifically for it. Would have been a nice twist if Lucas or Amy were competitive swimmers. In which case 2 min is no problem.
The average healthy adult can train themselves to exceed 2 minutes rather easily.