An all in rush nuke can be done in about 18 minutes, but if it fails your Eco is wrecked. A "fast" nuke is around 20-23 minutes, and there are almost always nukes by 30 minutes.
Depends on how good the players are, nuke launchers are usually built pretty late since if you waste that much eco into something that only pays off after a long period of time, you're gonna die. Also one nuke really isn't all that significant in Seton's.