Yes, its too long. I've played along the whole Colosseum every week for a very long time, collecting the mythic box and the crystals. Now I started, played some 6 hours yesterday, then abandoned it at the 12th Colosseum. I don't even care how many crystals are in the 6th extra Col.
Another painkiller could be to make easier the selection of the cards. The current filters on the assembling screen (color, sets, only available) are good, though a sort by rarity function could be an improvement.