I am a very new pipe smoker. For me there are two major factors...
Time - if I can't sit and enjoy the pipe, in a good place, with no interruptions or concerns, I won't do it. Sometimes I will smoke a number of evenings in a row and then I won't smoke for a number of days as well. Right now I have not smoked in almost a week due to my work and travel, but I'll probably fire one up today sometime! :
:
Enjoyment - at least for me, like any luxury, there's always a point when I have had enough. I just know it when I hit that point. Most of the time for me this is one or two bowls. Beyond that, I start to not enjoy it as much. However, on one occasion last month I took my pipe with me fly fishing and I think I smoked four bowls because it was just very, very enjoyable that day.
Beyond that health, finance and other considerations are out there, but these are the two major ones.
I can't at the moment see me smoking all day. I would never say never, but I live this way with other luxury items. For example a bottle of a really nice single malt will last me a long time for the same reasons. It is always special when I can sit and have a 'wee dram' as the Scottish would say! :D