When you start seeing spiders?
Check out the life cycles of the spiders in your area online. Temperature and timing.
I suppose it also depends on the product you are using and what kinds of residuals they provide as well.
and then how many treatments do they get? If it's one....that is different timing than if they get say a four treatment program.
I can't tell you when to spray because I am not from your area, don't know what product you use, and or what you are trying to control.
Google for an hour. You should be able to get some good information from local sources or university extension services.
How did you pass the tests for the license and you don't have a handle on this one basic thing?
When to spray.