cites both possibilities, but I think favors the position that it was a one-time-only deal.
but they did not continue They prophesied on that day only-Thus it is stated in Sifrei (Beha’alothecha 1:42:21). The Targum renders “and they did not cease” [meaning] that their prophetic powers remained.I could see how both are readable into the text on a peshat level. Certainly וַיְהִי כְּנוֹחַ עֲלֵיהֶם הָרוּחַ could carry the implication that it was only then, when this spirit of Hashem rested on them. But it could be read otherwise. Similarly, Moshe's reaction to Yehoshua a bit later might, but need not necessarily, be read as stating that these are more permanent prophets.
I am not going to get into this into detail here, because this is not the point of this post.
Rather, I had a fleeting thought which may well be nonsensical, but I wanted to record it here so that I could look back at it later to reconsider. Namely, how about placing the sof pasuk in pasuk 25 on the word וַיִּתְנַבְּאוּ? If so, the next pasuk would begin with וְלֹא יָסָפוּ. Then, it would refer to Eldad and Medad who were not gathered and thus remained in the camp. And thus they prophesied in the camp, for the spirit of Hashem rested upon them while they were still in the camp. Then, יָסָפוּ would not mean cease and would not mean continue. It would mean gather, just as the several instances in the immediate context, e.g. וַיֶּאֱסֹף in pasuk 24, וַיֵּאָסֵף in pasuk 30, and the examples in the next incident of the slav as well.