IIRC Gohan mentioned, either that Pan was born or that she was about to born when he fought that hench-meng. Considering that Videl told Gohan about it and was able to do the ritual, I’d think no more than 9 months.
I guess that could be enough time for Gohan to lose his Ultimate form. The scaling in RoF is whack. Gohan says that Tagoma is as strong as he was when he was at his best, and yet Gohan beat him with Super Saiyan.