OK. I'd run a can or two then drive the car for a week then give another can. Sludge builds up over time and won't dissolve completely. The first cleaning will soften sludge then driving will clear some away. You will then be able to use a second can to continue the process.

Think of a bucket that has mud on the bottom. it can be rinsed out but some will be stuck to the bottom. If you let it soak for a while then rinse again, more will come out.