Two separate things: a written workmanship warranty on the install, and the fan maker's warranty. Ohio also requires every proposal to state either a guarantee to bring the average below 4 pCi/L, with its conditions, or why no guarantee is offered.
More detail
The guarantee statement is the piece most homeowners never see spelled out elsewhere, and it comes from the Ohio Radon Mitigation Standards in the appendix to OAC 3701-69-08. Read its conditions; most are reasonable, such as keeping the fan running and the sump cover sealed. On the fan, the EPA notes that fans often last five years or more and that manufacturer warranties tend not to exceed five years, so expect at least one fan replacement over the life of the house. Ask whether the workmanship warranty transfers to a buyer if a sale is possible. Keep the closeout package, which Ohio requires to include the contract and warranties, the floor plan drawing, and the pre- and post-mitigation results. If the post-mitigation test misses the guaranteed level, the correction belongs under that guarantee. See what an Ohio proposal must include.