I did my leaking plug in less than 3hrs.  You can adjust bthe valves while your at it.  Let it sit overnight to allow the sealant to cure.  Then put 100-200 miles test miles to make sure it's good.

I'd never go far on fresh mods/rebuilds.  I tied that with an old MGB when I was 18yrs old.  Buttoned up the lower end at 1am and hit the road.  Spun a rod bearing at 3am and had to spend the night in the car on the side of the road.  Found a phone in the morning and waited till noon the next day  for my father to show up with a truck.