late response, but I'm pretty sure it took a lot longer for an uber velvet to spawn, as well with the hunter victory condition.
The Uber that spawns is randomized. Unless you only selected the Uber Velvet Worm, then there isn't any exact time you should be expecting a specific Uber to spawn. However, I do agree that waiting for an Uber to spawn is quite boring and tedious. I'm not exactly positive of this, but I believe that the spawn rate of Ubers is also dependent on the difficulty.
Anyway, It's worth mentioning the freeplay update is in the works. With new Ubers soon being added, perhaps there could be a option where we could manually adjust the spawnrate of Ubers, but I'd imagine such a feature wouldn't be highly requested or even needed.