First time I've finished the 12 basic Colosseums and 5 of the extras. I played the #5/6 extra only to know how long is it. (Terribly.)
Since then, I stop somewhere between 8 and 12 of the regular ones and fight only those extras which come handy for the event quests. It would be interesting to know if there are still 5 crystals in the 6th extra one, but not interesting enough to spend some two workdays finding it out.