Hi
@mertingen,
If you can give a location, the the time of the day can be calculated. With this more or less you can anticipate when she will switch to flowering.
https://www.growbarato.net/blog/en/when-does-cannabis-flower-outdoors/
The website like the above can give you an indication and you can prepare the plants by:
1) Flushing them more than a week
2) Starting flowering nutrients afterwards.
Note that any Nitrogen accumulated in the soil will negatively effect the bud formation. So flushing is important.
Hope this helps,
Happy growing,
Y