I like to play the largest size(40x40 I think it is) and I am sure it is possible to eliminate the oponet before the tech runs out. As far as plant improvements and such I also think they will last a VERY long time. Personally I have 1-2 planets which I do concentrate on getting ALL social improvements to construct some really nice planets, but most of my planets don't finish everything by the time I reserach all the technology(which is my favorate way of winning).
Keep in mind that SD plans to add many new technologys, ships, social improvements and events over the coruse of the next year(and possibly more, I don't know). But there are also going to be alot of modders creating their own add-ons to the game which can EASILY be done... the game will almost ALWAYS be fresh with new things to discover.
~SDC~