It saves semi-automatically.

Under Settings, Options, Setup, Numeric Detail Input you can set the Automatic Detail Save frequency, but even if it's disabled, exiting Lacerte normally or viewing Forms will save. 

I personally have it set to save every minute (as often as possible), and when I've had a crash, I generally have lost minimal input. The downside is if you totally screw something up (over-zealous deleting, anyone?), you can't go to Detail, Abandon Changes and undo it unless you notice really quickly.