I was also curious and searched and searched... The only decent reference I could find to an actual half life value is below. Basically it appears to be wholey dependent on the breakdown of the tablet in the system. Once broken down the life is very short. I would expect no much longer than 8 hours. Here is where the infor is from:
http://www.inchem.org/documents/pims/pharm/pim904.htm
Here is a short cut from the long extract on Mesterlone:
6.1 Absorption by route of exposure
The absorption after oral dosing is rapid for
testosterone and probably for other anabolic steroids, but
there is extensive first-pass hepatic metabolism for all
anabolic steroids except those that are substituted at the
17-alpha position.
The rate of absorption from subcutaneous or intramuscular
depots depends on the product and its formulation. Absorption
is slow for the lipid-soluble esters such as the cypionate or
enanthate, and for oily suspensions.
6.2 Distribution by route of exposure
The anabolic steroids are highly protein bound, and is
carried in plasma by a specific protein called sex-hormone
binding globulin.
6.3 Biological half-life by route of exposure
The metabolism of absorbed drug is rapid, and the
elimination half-life from plasma is very short. The duration
of the biological effects is therefore determined almost
entirely by the rate of absorption from subcutaneous or
intramuscular depots, and on the de-esterification which
precedes it (Wilson, 1992).