Get some cleanish, empty bottles, your biggest kettle, a thermometer, a stopwatch, and some safety glasses (hey, by the way, I'm not liable if a bottle explodes). Fill up your bottles with water but don't cap them. Keep them in the same space as your cider until you are sure the temperatures are the same. Fill the kettle up with enough water to almost completely immerse a bottle (or a few) that is standing upright in the kettle. Heat the water in the kettle on the stove to boiling. Put your safety glasses on. Start standing some of the open water-filled bottles (1-3 at a time) in the kettle, start the stopwatch, and keep a close eye on the temperature. When that/those bottle(s) reach 161F (or maybe it is 180F, I'm not 100% sure), stop that stopwatch, add 16 seconds to it, and record that. Then pretty much repeat the process with the capped cider bottles, put them in the kettle, wait as long as it took before, and pull them out.
And hopefully nothing explodes. Again, I'm not liable if something explodes and injures someone. You could've just used campden tables, so don't blame me.