I just got my three wins. It took about 10 days for all of them, and I condsider myself lucky.
I actually think a spawn timer is perfectly acceptable for these, given the magnitude of the mythic quests. The fights themselves are not overly challenging, and they can't very well buff the NM since it is technically meant to be killable by a party of 75's (although let's face it, it never will be again).
That being said, a modified, or WORKING, time spawn would have been greatly appreciated at this point.