According to Official ARK: Survival Evolved Wiki you need around 92 fortitude to even survive -5 °C naked. You are probably looking at 150 fortitude, or 75 levels (out of 100 for non-ascended players). Possibly even more. Investing so many points will render you quite useless in PvE and PvP.
Utility. Each point spent on fortitude will increase your knock-out threshold by 1. This also reduces torpidity gain rate and increases torpidity drop rate.