Hey mate,
Most lawn sprays are based on a repeat spray program. I am doing some chemical efficacy work at the moment for APVMA and they are all on a 28 day (4 week) repeat spray. You will get more efficacy if it is warmer as the stomata on the plant is more open and you get more entry into the plant cell. If you spray when its too hot you will get a chemical phytotoxic burn as its more of a heat burn of the chemical on the leaves. Ideal temps for spraying would be between 18-30c. Have you ever used a delta t table, you probably read about one when you got an ACDC licence? They can help you get the best out of spraying.
Cheers
