General rules for telling employer?

Hi everyone, so I’m still getting my bloodwork done but I’m estimating myself to be maybe 4-6 weeks. I’m trying to decide when is the appropriate time to tell my employer? I read through the handbook and it didn’t really specify just that it was enough time of them being aware for me to get the time off. When do you recommend telling your employer? Is just sending an email a good enough way to tell them?