It is a little longer than usual, but it doesn't necessarily mean you should worry quite yet. :) Everyone's ovaries are different and it's hard to say going into a cycle how someone is going to respond. Her RE may have started her off at a low dose thinking she'd respond well to it, but found out as they went along that she needed more - thus lengthening the stimming time. If thats the case, had they started her off at a higher dose, the length of days might have been shortened. There's really no way of knowing what the outcome will be just from the length of stimming alone. All in all, I wouldn't worry about it, but look forward to hearing how the ER and fert reports go.
I also wanted to echo MrsMcIrish's sentiments about how amazing it is that you are a gestational surrogate. What an incredible INCREDIBLE gift!