Well, I beat the game today, just playing an hour or two a day for a few days. I'd say it is a bit shorter than We Love Katamari. It also lacks the diversity of that game. You start to get a little tired of doing the same thing over and over the last few levels. Yes, he tells you to collect different types of things ("cold", "heavy", "big", "grand", etc), but mostly it is a race to beat the time/size milestones just to make it to the next stage. I think towards the end you have to do 4 stages (house, town, continent, world) though the world stage may be reserved for the very final level because I got much bigger in that one that I had previously.
When you beat the final level you play the 8-bit sidescrolling Katamari during the credits, which is very cool. You have 3 lives (and die if you fall in a hole or get caught on the left side). I didn't make it through the first time but the second try was pretty easy. Then the king said I had some sort of bonus level to try, but I had to stop (I can barely type right now 'cause of the cramping!).
I still have more to do in the game - particularly collecting all the presents and cousins, and the rest of the eternal levels (I got the house one first try). Then if I still feel like it I might go for a complete collection as I've come close on the other games but never quite did it. I'm not sure how much I'll replay levels to get better scores (unless I'm shooting for the eternal) because it doesn't seem to make a difference, unlike the console games (glowing planets and shooting stars for example).