Leaving always seems to take more time than going in.
Depending on how many people we have there, what stuff they've got, and how much of a power vacuum (and infrastructure) we want to leave behind, 60+ days seems reasonable to me. If we wanted to leave all of our infrastructure and most of our equipment in place, and divert all of our strat air to Syria, we could probably get everyone out in 30 days. Otherwise it's probably going to take a while.
I would rather the President have said, "In 30 days, I want a plan to withdraw...".