I have built and painted 7 knights and always needed about one weekend for one of them (including washing and cutting from sprue). But! I have invested in a good airbrush, oil colors and varnish for weathering and used a simple color scheme (house makabius, instructions found at battlebunnies). And I used to work about 36+ hours a weekend. (with an unhealthy amount of beer ;))
So, with an easy colorscheme you will be able to put a huge, good looking force on the table within 2 to 3 month of long weekends.
But playing with them is a really different thing. As already told, you will have to wait quite a large amount of time, because the rest of your side (always used them in large Apoc games) needs also time to move, shoot etc...
I also had the situation (in 7.edi) that some players wanted to refuse to play against them, but since the boons for masses in 8. Edi this ain't no problem any more.
So, long short story... If you want an impressive force within a quite planable time, knights can work very well (airbrush recommended) and they, As a full army are (at least in my place) a rare sight to play against.
Not the no-brainer army and against an used contrahent difficult to play, but unique and fun nontheless. So go for it!
(sorry for my english,it is not my main language)