You can start it immediately if you leave the radiator cap loose it can't build pressure and the heat will help cure the silicone.

 

Well I figured I didn't want to replace it again so I gave it extra long. I vacuumed it out which took about 45min since it obviously has not been done since 1985. I probably waited about 1.5 hours. I started it up and let it run for about 10min or so, no leaks so far. I couldn't drive it around since it has no brake lights or turn signals as of yet. I did figure out my defrost doesn't work. The heater does it just wont switch to defrost. Oh well that is a problem for another day. So far 2 of the four things has been fixed before its street ready. Now to figure out those lights and get a new windshield.