Figure on 3 hrs before flight time for DME pickup...give or take 15 mins either way. For a 6pm flight you can expect to be picked up between 2:45-3:15. You need to be at the DME bus stop, all ready to leave, no less than 15 mins before the stated pickup time. The stated time is the time the bus is due to leave the resort, not the time you should be heading to the stop.
You can check your bags first thing in the morning with RAC, which is nice. That way you don't have to do it at the airport. You can store any carryon bags with Bell Services while you're at a park. Be sure to leave enough time to get them when you are ready to leave the resort though. I usually leave about 20 mins for that...just in case.
So...if you have to be at the resort for pickup at 3pm, you should leave the parks by 1:15-1:30. That gives you plenty of time to get back to the resort, get your carryon bags, use the bathrooms, grab a snack and head out to the bus stop.
As already mentioned, a notice will be under/on your door the day before you leave. It will tell you what time your pickup is and also give you the phone number to call to pay for any checked bags.