Non qualified use is defined as a period after 2009 in which the home was not used as a personal residence. In addition, if the home was rented out, the depreciation which was allowed to be deducted as an expense against the rental income, is required to be recaptured and is not eligible for the exclusion. Please note that this is applicable whether or not the depreciation was expensed. The determining factor is the allowable depreciation.
There is no limit to the amount of time this exclusion can be used, but there does need to be at least a two years between sales.