Depends on when you customize them and which MBs you select.
If you select only the free MBs:
If customized more than 30 days prior to check in, they will start to process them at 30 days and ship shortly thereafter.
If customized less than 30 days prior to check in, they will start to process them the next day and ship shortly thereafter.
If you select any premium (paid) upgrade MBs:
They will start to process them immediately and ship shortly thereafter. You can't change or cancel a MB order if it contains any premium MBs. Also, if you cancel the room reservation, you will be charged $14.99 per premium MB ordered.