The following is shamelessly extracted from the following link.
Finally, the "average variance extracted" measures the amount of variance that is captured by the construct in relation to the amount of variance due to measurement error and can be calculated using the following formula: (summation of squared factor loadings)/(summation of squared factor loadings) (summation of error variances) (Fornell & Larcker). If the average variance extracted is less than .50, then the variance due to measurement error is greater than the variance due to the construct. In this case, the convergent validity of the construct is questionable.
I haven't used SPSS in some time, and I don't remember seeing an option to perform these calculations, but you can certainly do it using the syntax.
These two links give you an introduction to SPSS syntax. What I would do from here is examine the syntax SPSS gives you when you perform an FA, and use the variables named to compute the average variance explained.
Sorry for the lack of a definitive answer, but I felt that some response was definitely better than none.