I do mine every couple of times that I do a big project on my island. I usually want to do it every time I make a big change but most of my projects span over a couple of days with trying to get the right furniture and whatnot, so it ends up being after every couple of big changes.
I'll be updating it in a few days, as I have a few villager moving in, and because of that, I made some changes on the island. But other than that, I'll probably update it once again after I get the Christmas decorations and once more at the start of the new year (after removing the christmas decorations and maybe replacing it with something else in the main square).
I will update it whenever I see a cool weather/sky formation. I'll write that date down somewhere and if I make small or big changes to my island that I want included in my recent dream, I'll TT back to that cool date and update it again.