I think you would have to assume that if you were to start on the 310, you would be unlikely to get off it until it is retired. Given that it was to be retired years ago, and we have just firmed up on the order for 5 freighters (as I understand it) then i would suggest you would need to be prepared to be stuck on it for quite some time.
My advice would be don't accept the 310 unless it was a DEC position.
I would guess that sometime next year 330 F/O's will be offered upgrades, out of seniority, to go to the 310. At present Airbus F/O's are looking at 4 years to command (that's the senior ones), who knows for the junior F/O's.