can't speak for anyone else, but i will never stop buying records. at any given time i have between 2-5k on hand, but i trade and/or sell by the hundred or so - so my collection is always in flux. the most i've ever had at once was around 10,000 - i sold about half of those when it came time to move, and went on a nice vacation.
there are around 1500 12"s and 250 or so 45s that i will probably never get rid of, but beyond that it really depends on what type of digging and listening i'm doing. there are periods where i listen for 7, 8 hours a day, and then other periods where i only want to play a single album or two late at night. sometimes i'll go to 2 or 3 stores in a day and come home with 100, other times i won't buy a record for months and just dig through what i have, pulling out 'must keep' and 'must sell' and 'give away' as i go.
i do replace things from time to time, but rarely have doubles of anything - i keep the one that sounds better and usually give the other away (unless the version i'm selling costs a lot).