The Milorganite website generally recommends application every 6 - 8 weeks. For flowers specifically, they state, "apply Milorganite at planting and then in early spring when plants emerge and mid-summer after flowers bloom"
https://www.milorganite.com/ga...
I tend to do it less often, because I'm incredibly lazy, but I've read multiple places that it doesn't damage plant to over-fertilize, because they can only take it in at a certain rate (though it does waste money).
