I’m in orbit and have a leak that needs to sealed on the outside.
Would a tube of silicone caulk be handy here or will it “set” in about 1 hour? or should i just stick with duck tape? 
well, perhaps not a good question per recent news, but I think that both adhesive methods stated would evaporate away into the vacuum over time
If you mean silicone RTV (room temperature valcanizing rubber), they require air to harden. Or more specifically, the moisture in the air triggers the reaction.
I’m not sure about duct tape, but there are tapes that work in space. We use Kapton tapes all the time in high-grade vacuum chambers.
Here’s NASA’s External Adhesive Pressure-Wall Patch.
It’ll probably work better than tub and tile caulk.