I'm not sure, but it goes up by a set amount on each reset (1.5x, 1.4x, etc, shows on each bomb).  The next level will go up by that same multiplier and will require the full level to be completed in that amount of time.  The investors from earlier levels are earlier rewarded on the first level-up, so there's not real advantage to "winning" a level early in the process rather than later.  The lowest I've seen some of this start is ~.005BTC, but that also seriously limits the amount of winnings that are possible.