If, in lieu of getting an extra test each night as we've discussed, you prefer to stop the insulin, it can take up to six cycles to drain the depot. I have, however, seen a couple of cats who were very overdosed on high doses (e.g.10u) that took a bit longer.
If he goes high and stays there, we can fast track his dose back up.
As I mentioned before, this has to be your decision. Several of us who have posted have also tried to convey that leaving the dose and getting one extra test a night will give you more data to see what he's doing. we don't mean to confuse you but the reason we suggest that is so that if he does need more insulin, he hasn't gone down to zero. If he is overdosed, it will be evident fairly soon and then you could stop the insulin.
Either way, we will help you interpret the numbers once you decide.