It depends why you are dropping buy ins.
If you figure that you aren"t really focussed, are playing badly, etc, then it"s definitely time to take a break.
However, if you are getting your chips in good and continually losing to "bad beats", then what"s the point in stopping? I once read an article where someone said that life is just one long poker session.... ie if you are on a cold run, and take a break, you will still be on that cold run when you return. You may as well just play through it now and wait for things to heat up again. Probably best to drop down a level while you"re running bad though!
I"ve found it useful to jot down the big pots that I win/lose during a session, and note them as lucky/unlucky or good/bad... and if during the session the trend shows that I am losing big hands through being unlucky but playing correctly, I generally feel better about it.... but if it shows I am losing big pots through playing badly, it"s time to stop.
Also, I don"t think you should get hung up on losing 3, 4, 6, or more buyins per "session" - keep the longer term view. So long as you are playing well/correctly, and are up over the longer term, a single "session" can just be put down to variance (unless you are playing badly or making poor decisions during that session)