Depending on plant size in relation to pot size I either top dress as soon as I put in flower if it's a big plant and I think there's not much left in the pot to keep it healthy.
Then every week or two after until week 6ish depending on how long it goes, and I re use my soil so whatever doesn't get used this round will be available next cycle, so I'm not wasting any of my nutrients.
If they are just put into a pot recently or aren't that big a plant relative to pot size then I start my top dressings at week 2.
This has worked really well for me, last grow I got lazy and didn't top dress my bigger plants when I first put them in flower when I knew I should (was just being lazy) and they got fairly yellow, if I had just started top dressing at the start they would have been green the whole way.
They still yielded pretty good but would have done better if I top dressed earlier
Here's a pic you can see the 2 in the back left and center are pretty yellow, everything else is super green n healthy.
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Moral of the story top dress sooner than later

Actually now that I think about it, heard a podcast with Jeremy from build a soil saying you should top dress as early as possible even if your plant doesn't need it yet. I think he said It helps get your roots used to the top dress and your roots will know where to find the nutrients when it needs them. Was a long time ago so can't remember exactly what he said, was something like that.
Happy Growing