We should be able to make some approximations regarding sentience. If we only consider Homo Sapiens Sapiens, then yeah we only have one data point. If we consider other extinct hominid species, we have more. If we consider dolphins, ravens, etc - we start to have a fuller picture and can make some (at least slightly) informed estimates to that term
You're still extrapolating from a single life-creation event. Until we find a second planet that evolved life entirely separately from Earth, we have exactly zero meaningful information about the genesis of life. Without knowing what percentage of life is roughly similar to life on Earth, we have exactly zero meaningful information about how valid Earth-centric extrapolations are.
