Prior post is a good worst case, a little more data suggests planning 35 mins for each trip.
For the monorail, you will spend about 15 minutes actually travelling on the monorails (about 7 from Epcot to TTC, and about 8 from TTC to Contemporary). The rest of the time is waiting for monorails (worst case ten minutes each for 20 more minutes, for grand total of 35 minutes), waiting for second monos if you travel during very high demand times (e.g. at closing) and there is a line waiting for monorails, signaling slowdowns, etc.
The boat from YC/BC to MGM is about ten minutes. Add 5 for the walk, and 20 minutes as a worst case if you just missed the last boat, and you get to 35 mins.
Hope this helps.