I used to install those systems, and it's a couple minutes at best, if they've replaced the batteries recently, which they never have. It'll give them time to finish the transaction they're in the middle of ringing, then shut down cleanly, because the server in the back room _should_ have a bit more battery to keep the database consistent.

PoS systems aren't particularly power-hungry, but store owners never want to spend an extra cent, so they go with the smallest UPS they can manage. (And arguably if they went with a big overkill UPS, its after-outage recharging power would be larger so you'd be able to put fewer registers on a single circuit, so it's not as simple as just dropping in a bigger UPS.)