Upvote:2
There is no hard and fast rule, but in general you'll want to spend more time outside the US than inside.
Cuba and Mexico do not restore the clock, as stated by Timatic, and the clock will continue while you're in these countries.
So if you spend 60 days in the US, then 7 days in Cuba, then (for example) 3 days in Cancun, you will have 90-60-7-3=20 days left in the US