I think you can edit that number searching first in the aircraft folder there are many xsMU2B60_xx_xxx_prefs.txt (don't confuse with the "mu2b60_prefs.txt" one), choose the model that you normally use and want to modify (GNS, OEM, 5B, 4B, etc.). Open that file in a text editor and there is a field called "__hobs_time" with a number on it. Modify that number. I'm not sure in what units it is written but I suppose it's in seconds.
Hope it helps,
Jerry C.