What is the command to make a command block emit a redstone signal after 20 mins

You will actually need to build a redstone timer that takes 20 minutes to output a signal. There was a recommendation for a /wait command, however, it was not implemented. There are plenty of delay timer designs you can look in to though. I have also included a link to the official commands list.

Recommendation for /wait: http://www.minecraftforum.net/topic/1962496-wait-command-command-block-only-cut-down-curcuits/

The full list of commands is available here: http://minecraft.gamepedia.com/Commands

The easiest way to accomplish this is to have a command block summon an Item entity with an age of minus 15 minutes on top of a pressure plate on a block on which a redstone torch is attached to.

The item will despawn in 20 minutes, depowering the block and turning the redstone torch on.

There's no way to pause a command block in vanilla Minecraft. I haven't yet played it, but I believe you could accomplish this pretty simply using ComputerCraft, if installing some mods would work for whatever you're doing. An easier option, as imulsion points out, would be using the timer IC from RedPower.

Otherwise, since you need exactly 20 minutes, you could use a couple of daylight sensors (one wired up to be a nighttime sensor) with some non-sticky pistons to push some blocks to handle this. It doesn't have to be a very large or complicated clock.