How long can the data line for WS2812 Led strips be?

I am planning to install an LED strip with a 7m long cable for power and the data line.
Has anyone tried how long the data line can be?

My 2 cents. Your mileage will vary. Depending on the wire size, presence of inductors and stray RF, I have had setups shorter than 7 m become unstable. I found that attention to quality connections, especially at the joints, is crucial. I tried coaxial cable for one long run. It worked great, but it was too expensive. Perhaps a signal booster would help. You have to experiment, and pay close attention to the wire placement.

This may help.

I would suggest at that length you seriously look at a TxRx MAX485 pair for the data line.
Adds an extra conductor for data but eliminates a raft of data issues.
They’re cheap and very effective.