If you are using Disney transport, you should leave AK no later than 6:30 p.m. This allows for time to wait for an Epcot bus, ride to Epcot, walk from Epcot bus stop to main gate, go through security and the turnstiles and walk to the Land pavilion, arriving the recommended 10-15 minutes prior for your ADR.
If you are driving yourself from park to park you should leave between 6:45-7 p.m.
The Transportation Wizard on Our Laughing Places puts it at a 16 min bus ride...but you've got to factor in leaving AK, waiting for a bus (could be 20 min or so), bag check at Epcot if applicable, and the walk to get from the entrance to the Land. On top of that, you never know if there will be an event or issue causing traffic. On top of that, you should be arriving to your ADR about 15 min before actual time. So...that puts you at about 7pm if there are no other issues. So, like @Tuvalu noted, 6:30pm is probably a safe time to aim for. And if you get there on the early side, maybe standby will be light for Living with the Land and you can get in a ride before your meal. Alternately, if your group isn't too large...check in early and tell them you want to wait for a lower level booth.