My client and prospect base is broken into five distinct categories: (a) repeat clients who are good for lots of continuing sales; (b) non-repeating clients who have used my services in the past but will have little reason to need me again (this unfortunately happens sometimes); (c) referral sources; (d) people who have probably forgotten about me; and (e) long shots.
I treat each group a bit differently when considering mailings. The repeat sales potentials (group a), get mailings from me as often as every two to four weeks. They'll usually stay on my mailing list indefinitely. Non-repeat potentials (group b) stay on my list, but receive mailings about once every month or two. They stay on because they can easily fall into the next category, which is referral sources (group c). Many prospects will never have an actual need for my services, but I've tracked distinct groups of people who appreciate the fact that they know about my company and refer me to their own clients and prospects. These people generally stay on my mailing list for at least a year, and I'll only take them off the list after I've called and verified that they don't want to continue receiving info from me.
The fourth group (d) is my favorite one. These are the people who always surprise me. I've talked with them in the past, but (predictably) they've forgotten about me. Every mailing I send to them will remind them that I'm still around and that I still want their business, and explains additional reasons why they should use me. And every time I mail to this group, about a dozen people call back and make a point of telling me how they had forgotten about my services, but my mailing reminded them of a project they want some help with.
Longshots (group e) are my test groups. This group may consist of people who have posted their business cards on store bulletin boards, businesses which have just opened up and I think there's some chance of them needing my services, or groups of businesses which I'm testing to see if they'll respond to my mailings. These "long shots" will get about three mailings. If they don't respond after the third time, I drop them off my list. It doesn't make sense to throw good money down the drain.
Whatever the size of your list, you need to constantly monitor your prospects and clients to get the best responses to your mailings. It's an old cliche, but your list is a "goldmine" because you can mine a lot of gold from the people on your list.