12 month minimum but try to negotiate a 2 year warranty. Remember warranties do you no good if you don’t track and exercise them. Be sure to not only track on your turnovers but also on your occupied units.
For example, if you do a tub resurface with a 2 year warranty, track the expiration and have your PM or maintenance team get in the unit prior to its expiration, regardless of whether the unit remains occupied.
You don’t only want to look at your vacant units but also your occupied ones. Can’t tell you how many opportunities I’ve had to exercise warranties on occupied units, it’s saved a lot of money on an otherwise very costly line item.