For a 3:45 flight, DME will give you a pickup time of between 12:30 and 1:00...as duds said, about 3 hrs give or take 15 mins.
Here's the thing to remember though. That time you are given? Well, that's the time the bus leaves the resort, NOT the time you should get to the bus stop!! You need to be at the bus stop, your entire party with any and all bags in hand, no later than 15 mins before the stated time.
I have seen too many people running up to the bus stop as the bus driver closes the door. I have looked out and seen people arriving at the bus stop as we pull away..with shocked looks on their faces!!! Be there 15 mins early or risk missing the bus.
And you shouldn't just assume there will be another bus coming along in 30 mins...it may, or it may not. Depends on whether or not there were any scheduled guests at that resort. There are many times that the DME bus stops only once an hour. And that will put you at risk of being late for your flight, or having to arrange, and pay for, alternative transportation.
So..if you are planning on spending some extra time in a park, be sure to leave enough time to get back to the resort...at least an hour if using the buses.