There are people to this day that will still grind on dust2 and office for days on end it seems. It's probably impossible to know but I am sure there are people who have played those maps 20 thousand times.
Pretty sure I had well over 1k hours on de_dust2, de_nuke in practice/competition games alone (not public), and I considered myself a casual clan player.
EDIT: seems rounds were shorter than I remembered