How much time does it take to change head gaskets on a 4.3 liter?

Doesn't make any difference really what size the engine is unless it has dual heads that both need to be changed, of course. If you have the right tools an knowhow it's reasonably quick, but you are probably going to find that the head is pretty well siezed on the studs that come up from the block, so even if you can get all the nuts off without screwing them up, you will probably want the special gripping tools that will allow you to pull those studs and replace them with new ones. You also have to figure out if you have a timing chain holding that sucker on there, or a timing belt. If it's a belt you'll want to replace it anyway, but if it's a chain, they can be very hard to get off. Then you always have to wonder if you put the thing back on again properly, and you have to do it all without making a mistake that will probably drive a valve through the top of a piston. I'd seriously invest 20 bucks in the handbook if I were you.