considering that XI had the feature for what seemed like forever, My fiancee played it long before I had and said it was there as long as he played it.... Why would it be technically difficult?
Personally, I think we should have a tally on character selection with TOTAL playtime across all characters. Then when you log into a character, you can see total for character maybe with a class/job most time: example On selection__total playtime 56days 6hours___ On character Ahmira Soulrage ___48 days 17h, 48% Arcanist___ maybe by /playtime for that character and /tplaytime for total playtime cumulative.