Franceschini and Maisano's f-index (Franceschini and Maisano 2010) is designed as a complement to the h-index. It is a measure of the time-width of impact. It is the range of time for publications with at least one citation and is calculated as$$f=\max\left(Y_i|C_i>0\right) - \min\left(Y_i|C_i>0\right) + 1,$$
or the year of the most recent publication with at least one citation minus the year of the earliest publication with at least one citation (plus one to consider the time spent preparing the earliest publication). For most active researchers, this will likely be simply the number of years since they first published since it will most often be the difference in age between their first publication (which probably has at least one citation) and their most recent publication to get a single citation (probably published within the last year or two).