Hmmm... it depends on your set up completely.
If you have something similar to the thurston james model from the prop handbook then you should be preheating your oven for at least 20-30 minutes.
I preheat mine for a full hour.....in the time in between i clamp all my plastic and get everything else ready... molds...vacuum system..etc..
After that i usually only need to heat my HIPS for 3 minutes and it's ready for ultra crisp detail....
ABS doesn't take much longer...only 4-5 minutes. However, the ABS is not as prominent of a sag, so a quick hand in glove check to test the ripple should do it.
Be careful to not overheat though...those are just rough times...hand checking it will tell you if you have enough droop and play. If it's too hot, you'll probably break the seal...or have a really thin pull in some areas.
Use risers as well...or else you're going to be doomed with webbing issues for complex or demanding molds.
Having an assistant always helps, but to tell you the truth, i do it alone most of the time and end up with great results.
80% of the battle for me is getting the oven up to temperature.
Make sure it is insulated well also, if you're losing heat you'll never get desirable results.
