gtk:filter-list-model-pending (object)

object -- a gtk:filter-list-model object
pending -- an unsigned integer for the number of items not yet filtered.

The accessor for the pending slot returns the number of items that have not been filtered yet.

You can use this value to check if object is busy filtering by comparing the return value to 0 or you can compute the percentage of the filter remaining by dividing the return value by the total number of items in the underlying model:

(let ((percentage (/ (gtk:filter-list-model-pending object)
                     (gtk:filter-list-model-n-items object))))
  ... )  

If no filter operation is ongoing - in particular when the incremental property is false - this function returns 0.