The simple answer is that the more often you wax the better your skis or board will perform!
In general a hot wax should last 3-5 days of snow skiing or boarding. Rub on waxes tend to last about a day.
A hot wax, without scraping, prior to storing your equipment at the end of your holiday or season is also recommended.
DataWax manufacture both iron on and rub on waxes.