I would allow up to 30 minutes. As others have said, if they're only running one train and not enough folks get off at the DTD station, you may have to wait for more than one monorail (I've had to do this when the line wasn't long in DTD, but only ~15 people got off the monorail, so we had to wait for the next monorail to arrive). In general, I've found that it's faster to walk from Earl of Sandwich to the park entrance, but the monorail can be a fun thing to do.