How long should my stomach feel hot/look red? I got the Venus Legacy done at 3pm and my stomach is still red / hot at 10pm!

Venus Legacy uses HEAT (radiofrequecy) and magnetic pulse technology to tighten the skin.
As this uses heat, erythema / redness may persist for several hours but typically resolved by the next day.
If someone is predisposed to physical hiving / dermatographism, the erythema may be more dramatic.  If redness lasting over 5 hours, consider taking generic Claritin am and generic zyrec pm as these may reduce the redness.
When used correctly' blistering, long term redness, peeling, and erosions should not occur. These should be discussed with the treating physician if they occur as they are NOT expected.

I'm sorry no-one has answered this question! 
Photos would have been helpful.  Redness and the sensation of heat can be normal for hours after a treatment if you are very fair. I am assuming the treatment was comfortable throughout.
Normally we push the temperature to the threshold of uncomfortable and then we back off the temperature so it is comfortable. If this is what the provider did, then it should have resolved by morning.
If it was uncomfortable then you may have had a burn. I hope it wasn't that! 
Let us know how you faired.