Concept of  modelling time estimation of top assembly:

Each part takes various time to finish the modelling process. When they are assembled in sub-assembly, the time to finish of this sub-assembly will be determined by finishing time of longest one. It does the same when those sub-assembly are combined in the top assembly. I would like you to suggest about how to input this property on the custom property and let it calculates the max time ( by using equations or link to property) in order to estimate the modelling time for each project automatically. Please see the flow chart below.

The figure show the TOP ASSY takes 5 hours to finish. It is estimated from the maximum time among SUB ASSY #1, #2, and #3. The same way SUB ASSY #1A takes 5 hours where obtained from Part B (5 hours).