Your benefit as either former spouse or widow of former spouse will be determined by his earnings record and the age at which you start to take benefits. At any given time your entitlement as a spouse/former spouse is 1/2 of his benefit followed by an adjustment for your age.
To get an accurate estimate I suggest that you (1) get his Social Security number and (2) visit a Social Security office and have an SSA representative make the calculation for you. Or visit the SSA site as suggested above.
Scott