If no rent is due for a while, just agree with the tennant what date they would like to end, and make it clear it has to be cleared of their stuff by then for a new tenant to move on the next day.
If rent is due, then I would give them 28 days.
Do be strict, make sure that they fully understand that they lose the right to anything left after the agreed date, including trees/bushes.
We have had a couple of occasion where the previous tenant came back later to retreive more of their stuff after the date, much to the dismay of the new tenant.