15 years.
The gain due to depreciation is based on the greater of (a) the actual depreciation that you took and (b) the amount of depreciation that you were eligible to take.
Unless you mistakenly took too much depreciation, you were only eligible to take 15 years of depreciation, so that is what your gain due to depreciation is based on (for real estate, it is actually called "Unrecaptured Section 1250 Gain", not "recapture").