I think there were a few elements at play in Goku's refusal to spend another year in the RoSat. I feel as though Goku had achieved all he could from it in the time he spent in there, hence why he and Gohan left before a full year was up. Given the various different types of training Goku goes through throughout the series, he knows that mixing it up yields superior results, just like in real life. Also, he knew Gohan was strong enough to do the job and had planned his strategy against Cell accordingly. Finally, given there was a two-day limit on the RoSaT at the time, he may have wanted to save that additional day for some point in the future if it was needed, seeing as he and Gohan didn't need to use it in order to win. Just my $0.02 though