I had a roll of 35mm HP5 that I exposed in 1976 and processed around 1999 - it was in a box in the garage for all those years that in the winter is around 5C and summer around 16C. I guessed that it would have lost some contrast so I developed it about 30% more. It came out fine when processed, no appreciable base fog. My guess is that being kept coolish (though not in a freezer) helped.

I have no experience with HP5 frozen for years but I would think it would be OK as long as it never got wet. Maybe someone with direct experience with frozen HP5 can comment.