The amount of time depends on the price potential for the domain and how much information the buyer provides. If I am unlucky, I get a first name and a Gmail address and that is all. Then I don't know if I am dealing with a broke newbie or company executive.
But if I get a phone number, company name, address, etc. then I can try to find out as much as possible so I know if I am dealing with an end-user, broker, domainer, or middle man. I can also get some idea of how much they may be willing to pay.
I can spend from a few minutes up to an hour if I feel I need to so I can create a "profile" of the buyer. One of the first domains I sold was to a broker at SEDO that contacted me. I was offered $450 for a domain I paid $5 for here on Namepros. I was pretty happy with the offer, but I wanted to make sure I was getting what it was worth. Researching the buyer was useless, since it was a broker. That should have told me something right there, but I was new at the time. So I researched what the price and availabilty of the other TLDs were for this domain. Doing this I was able to easily negotiate the price up to $1,000. Too easily. I found out later the real buyer was a German telecommuincations company. I don't know how much more I could have gotten, but I'm sure it was considerable.
I don't regret the sale at all, but I did take it as a lesson to be learned.