If it is a 22r(e) and you aren't having to do anything else other than the HG. it shouldn't take you more than the weekend. If they are both 3.0's, it is going to take you longer than the weekend. It is a slow process, a lot of hoses and parts to take pics/diagrams/video of and place all bolts in order(diff sizes on same part) in the order they go with the part, etc. I spent about 24hrs tearing down my '90 3.0, but all of the prep work made it a breeze to put back together(5 weeks after tearing it apart), if I wasn't sure about a hose/vacuum line, etc, all I had to do was look at video and pics.

Since both of your rigs are having issues, you could try the HG liquid repair on one while you tear into the other, to try and prevent the other from going before you get one fixed.

While the heads are off I would recommend getting them rebuilt(burnt exhaust valves are an issue w/the 3.0's), and have the injectors cleaned/rebuilt. You will also need new head bolts, the 3.0's are one time use torque-to-yield, and re-using them will absolutely guarantee another blown HG. Either buy new TTY's or head bolts from Enginebuilder, as well as the head gasket kit. If you have a quality head shop in your area that works with Toyota's, you should be able to have them rebuild the head and shim the valves for you, and if you have the new TTY's or head bolts prior to the the shop rebuilding the heads, they can install the bolts so you don't have to remove the cams to install the heads, there is just enough room to get a socket on the bolt to tighten/torque. This basically makes it a bolt up set the timing and drive rebuild, rather than having to set the valves yourself.

Figure about $7-1000(if you have the injectors rebuilt) for a DIY rebuild. Also if you ever thought about running headers now would be the time to install, of course this will add about $700 more to your rebuild costs.