The average lifespan of Fremen probably isn't terribly long due to the incidental dangers of life on Arrakis. However, outside of that, what is the lifespan of Fremen? On one hand, their diet, water intake, and the routine strain of daily life are very different from most humans; on the other hand, they do ingest a lot of spice.
It is said by Alia Atreides that...
“without melange and it’s amplification of human immunogenitic system life expectancy of for the very rich degenerated by a factor of at least four”
... so by this I would assume that they can live well over the average life expectancy of normal people if they lived normal lives.