I'd say "depends".
condition (1 ) if the vehicle is older/way older (early 2000s ODB-2 type and older), w/o too much bells and whistles to begin with, and granted that all they ever need is just a quick "boost" on their weak battery, then yeah, 15 mins and then try turning over the engine.
condition (2 ) if it's around 2008 or younger, or with lots of electronic gadgetry (fancy euro cars), then the answer is a blunt "no".
carb'ed engined cars ok.
I don't want any blame/liability on me.
Q.