What is the half-life of M4?

M4, my own economic measurement, was at 20172.4 on 11/30/2012. 

When was it at half this amount? On 12/31/04, it was at 10089.7.  So the half-life is right at 8 years.  So we can expect it to be about double the current level in about 8 years, in 2020.  And double again in 2028 and then again in 2036.  Somewhere in that range is where the fun begins, in my opinion.