There's no time you have to wait to sell your house after buying the home and getting a mortgage.
You can sell immediately if you decide too but you shouldn't because you won't have enough equity built up in the home and you won't get a profit when you do go to sell.
It's recommended that you keep your house for at least 5 years before selling it so you can build up equity in the home.
Remember that when you go to sell your home after taking the mortgage out you'll have to pay the mortgage off in full and satisfy that debt with the bank.
You'll no longer be able to make mortgage payments to the bank if you decide to sell your home.
They'll want all there money since they'll need to transfer the deed to the new owner or bank.