I would not recommend someone tackle this themselves unless they really understand what they are doing and are generally very capable with this kind of stuff. Of course it can be done, many folks here have done retrofits with great success ... but the cost of failure is high. Cut in the wrong place, JB weld in the wrong place, line things up wrong, handle parts the wrong way, dremel in the wrong spot, and you have to start all over again with brand new expensive parts. It is highly precise and detailed work, and it is obviously why $500 labor is pretty standard for retrofits.
As far as how many hours, it really depends on many factors, including what mounting method is used ... Nathan's retro took about 12hrs straight of labor from start to finish. But that's with one of the best retrofitters in the business doing said labor, if one is on their first rodeo then it'll naturally take longer.
Reading a LOT of technical threads on hidplanet and talking at length with theretrofitsource will go a long way in mitigating any potential snafus.
