No you don't. You sign on for 23 or something similar.
To the OP; 2 years maybe enough time to get a tour in if there are any going on. That's not really the point and you mentioned return for investment. Do you think that 2 years service would be an adequate return even if you passed P Coy or might you you be considered a fly by night (no pun intended) piss-taker? I'd be very interested to hear the Dukes opinion on that.
I thought 2 years including a tour might be acceptable, if I was open about my plans with the Unit from the start?
To everyone else, what about staying in the TA during Uni aswell (5 years in total), and what bearing would this have on OASC etc?