It all depends on the demand. If you are prepared to move to the outer echelons of the country then you may get on one within a year, but I very much doubt it. Forget it in the Southern bases unless you have 3-4 years under your belt. Alot of first officers who were on my course 5 years ago are still waiting, but they aren't prepared to move to get it. The bottom line is, if you are prepared to move then bid for it. You can bid straight away, you don't have to be in the company for x number of years before bidding. If you don't bid you won't get, that's the bottom line.
EMB bases at present are INV, ABZ, EDI, GLA, BHD, MAN, BHX, EXT, SOU & JER
Edited for the 3rd time: I Put BHX twice, and didn't include EDI/GLA, or JER