That seems to be determined on an individual basis. When I joined I was on 100% from the word go but my sim partner was on 70% and 50% during training. The primary difference between us was prior experience levels. I think that most who are on 70% contracts get 100% after a season but don't quote me on that.
I was asked during selection if I would accept 70% part time and I said yes as it would have been ok for my own circumstances. I have no idea if that made any difference.
Don't know if things are different this time around or what the basis is at the moment but I would imagine that supply and demand play a large part in who gets offered what.
A bit vague I know but I hope this is of some use.