Right when Robert becomes King, right before he marries Cersei and after Lyanna died.
Well you haven't seen the other stats ive written, but they have more than double the manpower of the next largest, and quite a larger army
Well on second thought I may have them already be married; I thought it'd be interesting to start before that but, it doesn't add much.
(Or the Reach. Even if Margaery is probably really young at this point.)