The main car that I was wondering for was a Nissan Altima 2.5l 4dr. 2007 with 138,000 Miles
It depends on many factors like operating revolutions, engine performance, manifold location (cooling), etc. If you are concerned about overheat - don't worry, hot manifold is okay. Some high performed engines can heat a manifold to yellow hot. Car engine could heat it to red hot. If your concern is safety of other components of a car, you can wrap your manifold in a heatproof bands, or make a shield of 2 - 3 layers of a thin aluminium sheet.