It's hard to say, the full memory loss probably takes at least a hundred years, if not more, since none of the gems or lunarians brought up the issue of old Goshe and Morga's memories even though they were the most recent captures to the moon.
Given that the gems will ((hopefully)) remember to feed him sunlight once in a while I don't think we'd have to worry about any severe memory loss at this point.
That also brings up the mystery of whether Paddy suffered any memory loss since he spent presumably hundreds of years covered by a cloth under a bench without any direct sunlight on him for large amounts of time. He was inconsistently brought back for a while each time Rutile did a successful surgery but seemed fine memory wise when Phos first meets him + on the moon, so I'd take that to mean that, again, as long as they remember to feed Phos once in a while, he should be okay.
Though youre right in that it would hopefully not take years for either the lunarians, slugs or Euclase to rescue him and put him back together.