Stan, the light doesn't need any time to warm up. As I stated, within an enlarging session whether it is one hour or eight hours there is total consistency with not only the enlarging light (electricity) but also with chemicals. If I increase exposure 10% the prints will look slightly darker, If I increase contrast on half grade they will be slightly more contrasty. From my experience if I made 100 prints they would all be the same whether they were exposed one right after another or with an hour in between.
It is when I print in another session with a different batch of chemicals and what other variables enter into it when I see differences that do not seem to make sense.