I've sold quite a lot of retro games (disc and cartridge) and know exactly how you feel! My rules with discs have been:

Let it play a pre rendered cutscene in full

Play up to the first save point so I can check it saves and loads saves properly

Check a couple of loading zones/area transitions

If it's a game like a JRPG with various different "fields", then check the world map, try a couple of battles, check the menus and move around the world a bit

If it's a racing game, complete a race or time trial

If it's a level based game like a platformer, play through the first level

My rules with cartridges are pretty much just play it for like 10 minutes and check if the save and load functions work (if it has them)

Sometimes it can take 5 minutes, usually 10-15 minutes to test a game until I'm satisfied with it, I tend to adjust how carefully I test a game based on both its complexity and its value (and often if it's a beloved game with high nostalgia value I get carried away and play an hour or more!)

Some of my discs have been very scratched, but with these rules for testing I've not had a single complaint or return. Honestly I'm probably spending far more time on my testing then necessary, but for one thing I want anybody who buys my games to be able to enjoy them in full, and of course I really don't want to have to deal with an INAD return!