1. each route can take up to 10 hours (talking about muv luv extra) unlimited it's the same route but the ending will be a little different depending on the girl you pick plus some different eroge scenes, I don't remember how long it is but it is pretty long.
2. Do all of them because in alternative you will understand more things if you do all the routes (it's not necessary but you will get more things)
3. Yes, like in any other vn you can replay and chose another route.
4. In muv luv extra? No, nothing, none, zero, just some cheesy love comedy, in unlimited mmm yes maybe. In alternative? FUCK YES.
5. Depends on you but once you finish alternative there is nothing else to do..
BTW, it will take oround 40-50 hours or a little longer (talking about muv luv extra, unlmited and alternative) for the plot to get interesting and good, yes even alternative in the first 10-20 hours...it is really really boring but then..oh boy.