how much oil needs to leak before the check engine light is on

OBD and OBDII (the diagnostics system responsible for 'check engine' lights) were designed around reporting emissions problems, so things like transmissions or engine oil level just don't factor into conditions that can set a DTC. If you're car doesn't have an oil pressure sensor you can get an aftermarket one, but honestly the best thing to do is get in the habit of manually checking your oil level on the dipstick every other fill-up.

My 2015 Subaru Forester has a low oil level sensor which Subaru installed due to these cars being oil burners. When my first engine went out the light went on at about 1.5 to 2 quarts low.

A 2010 Subie does NOT have a low level warning system...just a low pressure warning light which, as tennishoes noted, is usually too late to prevent rod bearing wear. The 2.5i will run with only 1.5L in it, which leaves the level BELOW the bottom of the stick! If your level seems dangerously low you'll still probably still beok if a max of 2.5L added fills the motor. If you needed to add 3L+ to fill to top dot you might've done damage. If your motor starts knocking at idle or upon accelerating the damage has been done...but not necessarily fatally. Just keep that bottom dipstick dot-hole wet at all times, eh?!