I put my ARH's in with engine down/on it's mounts and stock k-member still in. There were a few bolts that took a long time to get in, due to not being able to get to them much, but had I taken the motor mounts and brackets off, it would have gone much faster. Took me about 8-10 hours, doing it all myself, and taking my time, over 2 evenings, one evening per side.
Sometimes you can have clearance issues with the steering shaft, making the install process take longer, too.
For the ARH's, removing the starter was needed to get it up in the engine bay with enough room to move it around into position.
Dont forget the time you'll need to hook up the header to the rest of the exhaust. Unless you have the pieces to bolt in place up to the mid-pipes, you'll have to cut/fit/adapt/etc. to make things fit up to the headers.
If everything goes smooth, it can be done very quickly indeed.