Sforno on Exodus 18:2/ספורנו על שמות י״ח:ב׳

1אחר שלוחיה. After she (Tzipporah) had sent a message to him to find out where they were encamped. Moses had informed her that the Israelites would not stop anywhere for any length of time until they would reach the mountain of G’d, Mount Chorev, where they would serve the Lord. She must have known about this as Moses must have told her of G’d’s prediction in Exodus 3,12 that once out of Egypt the Jewish people would serve the Lord there at that mountain. This is the reason why it took Yitro so long before he came as he had to wait until the Jewish people would make camp for longer than one night.