The general rule says that it's always double time that you put in lengthening.
So we go like this: 1mm per day which means we get 3cm in 30 days. You double it and you have 60 days.
It's rule for young people mostly, if you are older then consolidation time can be three times the time you spent on lengthening.
But ofc, it really depends on a person or if any complication occured.
Btw, why 3cm?