Is there a way to approximate Hold'em blind increases to time a game?
One way to help this is to think about it backwards, so you have 8 periods of blinds (15 minutes x 4 x 2 hours) and let's assume 8 players, 100 chips each.  So, ideally, you want one player eliminated each period.  What's a high-enough blind to encourage play?  Try 10-1 or 20-1.  With 10-1, the last period will have 2 players, each with 400 chips, so that's a big blind of 40.  The second to last, has 3 players with ~260 chips each, that's a big blind of 26.

10 11 12 16 20 26 40 is therefore the mathematical progression (which we'd tweak to 5, 10, 12, 16, 20, 30, 40 for poker sanity and ease).  This is quite aggressive, so feel free to tweak the ratio.  Still, it should give you a method for determining good numbers.