If the devs wanted people to do this more than once a week they could have:
a) give a reward each time you set a personal best (that beat the timer). Since you have to beat your own best time there is diminishing rewards.
b) have multiple challenge rifts available at the same time -- one for each class, each lasting a week, each changing on a different day of the week. This turns them into daily events, but doesn't punish the person who only plays a couple time a week.