There are no hard and fast rules for this. Few points to keep in mind
a) Depends on how you are selling them (parking and inbound inquires) - in that case, one of them is that if you have held them for x numbers of years you got no inquiry, while other names held over same period of time are getting inquires, it is a good indication that it is time. If the buyer cannot think of your name (and there are thousands out there) then that is a good indication.
b) Keep doing research in Google and see how your name / keywords are trending - higher the results, good chance that someone will think of a name. This is really helpful to realize the value of the name. Strongly recommend extensive search on the name to realize the full potential of a domain name.
c) The offer that you got on the domain is no indication of the value of the name.
Even though domainers are most creative and can think of all possible uses of names, but if you run your portfolio through someone, they may offer a different perspective. It is quite possible that you discover more value of the name than you previously thought. A second opinion is always better and in cases of domains, multiple opinions are needed.