Battery Size for a UPS
62 watts at 12 v -> 5.17 amps.
85% efficiency for converter -> 6.1 amps.

6.1 amps for an hour is 6.1 volt amp hours (VAh).
You need to look at the VAh rating of the battery to determine how long it'll run your devices.
A 100 VAh battery will run your stuff for 100/6.1 = 16.4 hours before it is utterly depleted.

However, you never want to run a car battery down below 50%, they tend to suffer irreversible damage, and won't recharge well. 6 to 8 hours max is as far as I'd be willing to go, and if you use 8, expect shortened battery life. For hard use, you'll want to consider a deep cycle battery. They're designed for the sort of abuse a 6 amp constant load can supply.