how long and how difficult is it to fix a head gasket leak on a 99 dodge ram 1500 v6 3.9L

If u have all the right tools & know just what to do I'd say about 6 hours. If you've never torn an engine down & put one together then it will be challenging. Torque specs r very important for internal engine parts. I'd go ahead & give myself a weekend just in case l ran into anything l did not expect. Also expect to make a couple runs to buy tools if you've never done something like this before.
In short, for the average joe it will take one full day & will be a decent challenge. If u do this stuff all the time, 6 or maybe 4 hours & pretty easy compared to many other engines out there.

if u got a good shop to work in & good tools allow at least two days for this because u need to take the heads to a shop & have them checked for cracks & warping before u reinstall them,and that can delay u a good day,or more depending on how busy the machine shop is that does it,so id allow two full days to do this one,if not more,i got a 5.2 took apart right now & their saying a week on the heads for it,its not to difficult to to do if u have the right tools though that makes all the difference in the world in doing one of those,good luck.