The IRS states, "A fair rental price for your property generally is the amount of rent that a person who is not related to you would be willing to pay. The rent you charge is not a fair rental price if it is substantially less than the rents charged for other properties that are similar to your property in your area.".
There was at least one ruling that if the rental price was equal to 80% or more of the fair rental value, the rental price was considered to be "fair rental value". So, "substantially less" appears to be more than 20% less than fair rental value.
