It depends; how early to want to get to the theater? Some folks like to get there early, while others like to get there as close to show time as possible. The general advice is to allow at least 45 minutes to get from one place to another using Disney transportation. This allows for travel time, stops at other resorts, plus time to wait for the bus. I like to be at the theater at least 30 minutes before the show starts (to avoid the possibility of being late). So I would probably leave for the bus stop around 5:30 pm to be safe. I'd rather be early and wander around DTD for bit, than to be late or have to rush to get to the theater.
The other alternative is to take a taxi. It might not be faster (if you get lucky and walk up to the bus stop as soon as the bus arrives), but it probably will. The cost would probably be about $20-$25 plus tip.