Shoot, I really don't pay too much advance or post-trip attention to times, but we've been fine getting to the MK for rope drop on the big boat direct to MK. I think we left our camper about 45 minutes prior to RD and stood on the dock in line and waited about 5-10 minutes for the boat to arrive...so with 5-10 minutes of walking time I guess that's about the 30 minutes that Stacyrhood mentioned (now it is clicking in my brain!) We arrived at the gates before they started letting people in through the scanners, plenty of time to pick a line and get in and pick a good spot for watching the show. I do think that if you're planning on taking the first big boat that you'll want to be there early enough to get in line to be sure you make it on.
I love, love, love going for rope drop. We're morning people anyway, so it really is sweet to be able to have the park to ourselves (+ a "few" friends) and there's just something special about mornings and being up to see the dawn. DS was the first person on Splash Mountain on December 27th...tell me that rope drop isn't worth it! For this early bird it is way better than spazzing out and logging on at midnight for fastpasses!