IMO plugs should easily last a season, and for me that's a pretty long season.
I remember when I bought my first Doo that somebody told me I was going to go through a lot of plugs, so I bought 6, about 18 months and 150 hours later, I sold the ski and there were still 2 new plugs inside the little plastic carry cases.
When I bought my yami 1300, the owner told me that I will be going through plugs all the time, I bought a full box, I've put almost 90 hours on the ski and I haven't changed them yet.. stupid me for listening to him.
However he was one of those people that let their ski's sit for weeks at a time, but tend to go out into the garage and start the ski for 15/30 seconds every week, thinking they are doing the ski some good, are much more likely to go through plugs at a faster rate, since they never really get a chance to heat up in that short period of time and will foul as a result.
Just a thought, I don't start mine in the garage just for kicks very often, so when my ski gets fired up, its usually going for a 50 mile ride.