I have a Freestyle Precision Neo ketone meter and never wait 2h after feeding to test. For me, I decide to test blood ketones when my kitty's BG is high and/or if he seems "off" in any way (this could be vomiting, not eating, looking weird, whatever).
I will leave it up to the more experienced members to advise on how often you should test. I was doing it daily but have slowed down a bit. Depends on your kitty but like I said, I'll leave that to the pros here!