For story-based games I would at least expect four to five hours with a leisurely playstyle. Maybe less if they're inexpensive, definitely more if they cost over $20 USD. I'd obviously prefer longer games, but not at the price of artificial lengthening like grinding or excessive backtracking.
Aw. I just got Prototype 1 during Steam's Halloween sale. I wish I had waited so that I'd only have to pay $2.50 more to have the second. Oh well, I suppose my backlog is big enough as it is. :P
Amazing deal though.
New deal is the Megabyte Punch alpha, with a Desura key. Soundtrack's included if you BTA. Gameplay videos remind me of Super Smash Bros.
My key's somehow recognized but not currently working with Desura, according to the client pop-up. Might be an isolated incident. Sure it'll be fixed soon...