Go by the moisture of the wood. Time depends on the season, your local climate, how much airflow the stack gets and the species of wood.
You want to leave it on the stickers untill it wont dry any more, that will probably be between 10 and 15%, depending on your climate.
Then it's safe to flat stack it for long term storage.
In the summer that may only take 3 months, but if you stack it at the start of winter it may not be dry untill next summer.
Ian