If you fall asleep earlier in the day, you're paying offf your sleep "debt" right away. When I use it I go to bed at roughly normal bedtime the following night. At that point 8h sleep is certainly not enough. Though I'll usually not "catch up" more than 1-2 hours extra that night. If I want to I can push through and continue as normal, and even do multiple more cycles, but over time I do end up tired enough that I need to catch up with more sleep.

I tend to prefer to use it to offset sleep for relatively short periods - e.g. get an extra night of work, and then sleep a bit longer for the next couple of nights - as I feel it's more productive to get a longer block of time where I'm fully alert, and give up late evenings when I'd feel tired anyway to recover.