Vetsulin is an insulin that usually peaks (nadirs) around 4-6 hours after giving the dose. At the very minimum, I try to at least get a test in once during that timeframe. The problem with Vetsulin and with cats in general is every day can be different. One day your kitty can have his nadir at +4, and then the next day it's +6. For this reason, I do try to test at different times of the day to see how my buddy is doing. I will usually try a +2 or +3 (to see if it will be an active cycle or not), and then get a +4 or +5. I'll grab a +7 or +8 on occasion as well.
Really at the end of the day, just try at the very least to get a test in during the +4 to +6 timeframe when you can, but go ahead and move things around every so often to allow you to get to know how your kitty is reacting to the insulin.