In my class (last December) the 757 went senior. It varies every class but generally most people seem to want to avoid EWR so it does tend to go junior. If you want IAH 757 it will probably take a year. If you want to be in Houston, you should definitely bid the 737 because it will happen far more quickly if not immediately.
If you live near EWR, the 757 is definitely the best choice. Reserve B is not bad at all. The junior pilots are always called first so once you get a few numbers below you, you are not called much.
I haven't flown more than 12 days in a month since March. I rarely get called on short notice and the trips are very predictable - almost exclusively Europe with an occasional Florida turn or maybe a 2-day Vegas. Of course, with Europe you don't work until the evening and you are home early afternoon.
It is a very good quality of life.
Good luck.