Depends what you do with it. For basic uses it could last until the machine dies pretty much, for more intensive uses who knows. CPU progress has stalled significantly, main thing holding it back there might be stuff requiring quad core or higher end mobile performance. GPU would be the real question...but there's so much more mobile stuff out there it could last a while as a target. 8GB RAM should be fine, storage could be another thing to consider, but if you have modest needs there it's also not a big deal. Can't really upgrade it (without paying out the ass for Apple's SSD blades) but worst case there's still external storage if necessary.
Again depends on your uses, but should be fine for a while, particularly if it's meeting your needs now. If not (like if you want the 15" for a discrete GPU?) then I guess that answers the question. Update to update there probably won't be any single huge performance reasons to upgrade, barring something unexpected like them making a high power 13", which would be another one of my fanciful Mac dreams.*
*Wacky lineup idea I saw postulated once:
12" and 14" MacBooks, latter for people that just want a bigger screen, like the 12" and 14" iBooks way back.
13" and 15" MacBook Pros for more performance, upping the 13" power budget to allow quad core CPU w/Iris Pro, and 15" keeps its discrete GPU.