Hi, and welcome to FDMB
I was advised by the vets that it takes a few days for a dose to 'settle'.
If the acting time is 10-12 hours, I don't understand why it would take several days to see a response.
There do seem to be different schools of thought on the 'dose settling' issue.
My vet also said, at the time of my cat's diagnosis, that doses of insulin needed to settle in order to see the full effect. And I can see that there are some good reasons for keeping a dose the same for while:
In a cat that is
newly diagnosed it can be helpful to keep at a given dose because it’s impossible to predict how the cat will respond to the insulin, and that response
may be quite erratic initially (possibly due to factors such as glucose toxicity, as many diabetic cats will have been hyperglycemic for a long time prior to diagnosis).
A dose of one unit
may seem to do nothing or very little on day 1 or 2, but may suddenly seem to have an effect on day 3 (for example). Or, the blood glucose numbers may be very ‘up and down’ with little consistency when the cat is first on insulin. Every cat is different. But it is helpful to try to see what the
pattern is on a given dose before changing it. (Although an exception would be if the cat’s blood glucose was dropping too low, in which case it should be reduced immediately).
Insulin type matters too: Folks here have found that the long-lasting 'depot' insulins like Lantus and Levemir
do need time to show what any given dose, or dose increase/decrease, will do. (It can take a few days for the 'depot' of insulin to establish in the cat's body).
Once experience has been gained about how a given dose of insulin works, it becomes easier to make dose adjustments. And in fact some cats, like mine, do best on sliding scale dosage, where the dose is adjusted according to the blood glucose level immediately prior to the shot. So, he can get a slightly different dose each time I give the shot (in which case that dose has no time to settle at all! But that seems to work best for
him 
.)
Some folks here have also commented about their cat having slightly odd blood glucose numbers immediately following a dose change. I’ve not experienced it with my own cat but the phenomenon is so familiar here that it has been given a name: ‘New Dose Wonkiness’…. I don't think the term will ever appear in a scientific journal, but if you do a search of the forum you'll find many references to it here...
Eliz