It depends on how often you're injecting and if you want to capture a peak or valley if they exist. As mwhities suggested, test cyp peaks T blood serum levels on average 3 days post injection.
If you're injecting once per week and test 3 days after the injection, you'll likely capture your peak level. If you test the day prior to your next injection, you'll capture your lowest level, since you'll be injecting and increasing your T levels the next day.
If you inject twice per week or EOD, there will be very little peaks/valleys. I inject EOD and typically get my blood work done on days I'm not injecting.