In ten minutes you will wish you had never seen the stuff.
But seriously it depends on the bio load / poop load in your water . 3 weeks it could be time to remove some . Certainly 3 months it’s a problem.
Regardless of what people say it is possible to get it out of your tank if you keep at it every time you see just one little bit remove it.
I started out with like 10 little bits from plants. I got it in March or beginning of April and it still is not even close to covering my 5 at all. Maybe 30-40 plants swirling around. I expected it to be way faster than this!
It does not like getting its leaves wet. You write “swirling around “ It’s almost possible to kill it with high flow and surface agitation
But it can handle being under water for days and still be alive just.
To get more growth StarGirl make a barrier with a circle of air line.
I am sure you have seen Foo the flowerhorn videos ?
I had lights on my tank for more than 12 hours and kept tank warm like 80F all winter.
Grew superfast took out a few pounds over winter.
Started with 3 pcs, yes 3 hitchikers literally.
I now have lights on much less and have led lights instead. Mostly bright window sun.6 hrs
Tank is more like 78.
Same setup, same creatures. Duckweed barely reproducing.
"Duckweed" BBBAAADDDD! Some folks it grows like "Crazy" other not so much.
Purchased some "Plants" awhile back, thought I did a "Good" job cleaning them and even QT them for a couple of days. Some "Duckweed" made its way in the tanks. Let it go and as mentioned they multiplied times 1000. For weeks I was removing handfuls upon handfuls. Got it under control and finally out of my tanks.