General rule is no longer than about 7 days. However, damage can occur in only 24 hours. Damage could include plaster popping off or delamination, spider cracks, tile popping off, floatation of the pool, etc. These items are unlikely but depending on the age and condition of the plaster they could occur. Ideally you should fill the pool back up as soon as possible.

Our experience - we used to drain our pool every year due to hard water issues. We would refill the same day, which took 24 hours to fill. Always did it in the early spring here in Las Vegas. 6 years ago we drained it and left it empty for 5 days, then refilled it. Within a week we noticed delamination of the plain white plaster in many areas that got a lot of sun, not down to gunite, but rough areas where part of the plaster fell off. We lived with it for a year and resurfaced the pool with a mini pebble. Haven't drained it since, but due for this year and we will refill immediately. Barb

Pool company has left our pool empty for over a month, I cannot get them back out to finish the repair work they started - they drained our pool and now it appears all the plaster is cracking.