...once a year.
I find my mango beers suffer from a slight cloudiness, but I don't worry about it. They do tend to clear after a month or two in the keg. If you can't wait that long and worry about a slight haze, then follow the others' instructions for pectinase.
Oh and for how I add it, I just mash up the stuff in a blender and add to the secondary, stringy bits and all. After a week I keg, and find the mango bits floating on the top of the beer and reasonably easy to rack from under it. The mango even works pretty well as a filter.
Other bit of advice I have is NOT to use R2E2 mangoes. I did this last year (and so did the Guru) and we both found an unpleasant seafood aroma that developed after about a month.
Berp.