I did search before posting and none of the threads had any info in them. Just people saying the estimates were way off.
Maybe you never noticed this thread. viewtopic.php?f=8&t=3435
A look at the attached file from a spreadsheet should give you some idea as to why estimated times can be so different from actual print times.
My only observation is that the complexity of the print is a major contributor to extended print times and it is almost impossible to calculate an estimated time from any known factor or group of factors.
Not much help I know, but that's the way it is.