That means the counter will be reseted after 90 days?
Nowhere we said the counter will be "reset". Just that, once you exceed all your available activations, if MORE than 90 days have passed since the last time you got an activation, the oldest one will be reclaimed to grant you a new one, without having to contact us.
Of course, if you always use the DEACTIVATE function of the Addon Manager, it's impossible to consume all your activations because, even if you have hardware failures, the 6 activations are a buffer against unpredictable events (unless you get 6 hardware failures in a row...) provided, of course, you remember to DEACTIVATE when you are PLANNING an major update.