The official line from Disney is 2.75-3 hours ahead of flight time. Each resort has 2 DME pick-ups per hour, exactly 30 minutes apart. So some resorts have pick-ups at :00 and :30, some at :10 and :40, and some at :20 and :50. So while the exact number of minutes between scheduled flight time and scheduled DME pick-up may vary, it'll be darn near the 2.75-3 hour mark.
And by the way, in case you read about DME changing to 4 hours in advance of flight time, that was a TEMPORARY condition that already passed; it was in the day or 2 or 3 after the foiled UK terrorist plot hit the news. 4 hours is old news, no longer accurate.