In the 20 months I have been publishing here it has changed often. Lately it is once a day because I am constantly looking for something to share. When hubs are posted to my facebook group I always come into Hub pages to read and leave a comment.
Lately I am finding more on my Google + account because I receive two "daily newspapers" published by hubbers. If I find anything of interest I will pop over to read the hub.
For some reason I am answering more questions now too when I am on hub pages. If I feel I have something to add worth reading I may spend an hour or two answering questions.
One of the reasons I believe I do this is sometimes a question will inspire a hub or an article for my website blog. I dislike depending on Wikipedia for anything. There are rare times I can't find a hub which requires me to use it.
My final answer is as many times a day as I find something of interest through my social media, then once a day to look for something to share and answer questions. It has been as high as twenty and as low as one.