Repotting should be done when the plant has filled its current pot with roots. That is the main idea.
A clear sign is when you see roots coming out of the holes at the bottom of the pot. This usually means the pot is getting too small.
Other signs can be that the soil dries very fast, or the plant stops growing even if everything else is fine.
You can repot earlier, but in my opinion there are too many risks if you do.
So yes, waiting until the pot is well rooted is the safest and simplest rule.
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