I think the answer is variable based on use. Not "how many times" necessarily, but how are you playing it. A family playing recreational kubb could easily have a set for a very long time. The other end of the spectrum is the hardcore competitor throwing 300 batons a night.
Other major wear factors include leaving it out in the rain, or using it on concrete. This will ruin a set potentially in one game.
Overall, I'd say for a hardcore player ONE set would last ONE summer.
An over the top example is mine. I acquired 6 kubb competition sets for practice this summer. Half of them (the softer wood) are ready to be replaced. The other half I'll probably replace the next season. I think I figured that I will cycle through 3 sets a summer. Now, these pieces are not too dinged up for play, but they are more dinged up than what will be experienced in a tournament.
As far as common sets used, I'd say more tournaments were using OTG sets in 2013, but based on Nationals, JP may have a higher count of sets in use. I have experienced from 2012 to 2013 the trend of transitioning to JP's Backyard Games sets by many tournament directors. I don't know what the future holds, but I can tell you that 99% of the tournaments in the Midwest currently use either OTG or JP sets. I don't see that changing for a while, but who knows...