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.
AT least as long as the navigators if they choose to undergo the process of spice saturation. It's never brought up per se but inferred that all houses are human and the spacing guild just mutated humans from the 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.