I'm unaware of any previously published data on this subject. Note that this doesn't mean that there isn't any. It seems like you're asking the wrong question

I'm unaware of any previously published data on this subject. Note that this doesn't mean that there isn't any. It seems like you're asking the wrong question here. Are you trying to do market research for a potential calendar application? If so, I suggest you do market research. Hire someone to check your surveying methods, and ask a random subset of your audience the questions you wish to know. Then, multiply by (audience population / sample size) to get an estimate of the number of users. Also, it's customary to add "Ask HN:" to the title of posts like this.