Well to be honest, if Bryce really could tell how long a render would take, and be accurate, I probably would find another pastime. It's the unknown factor sometimes that keeps me thinking............ just a few more hours.
I do most of the rendering over night, so it doesn't tie up the PC so much but the last render was more of an experiment and so it was more or less continuous. I'm glad I did it though.
I guess the more one wants to get out of it, the more one will have to wait. But for me, if it takes too long, I will usually give up on it.