How to keep the camera flash on all the time and for how long?

The flash of a professional camera can't be on all the time, because of its working principle, and then it can be started instantly after storing electricity (some are special diodes). If you want the camera to have a constant lighting source, it is recommended to start with led lights, which are also very cheap, with rechargeable batteries and dry batteries. Always-on led lights can be directly plugged into the camera's hot (cold) boots. As long as it is turned on manually, there are several kinds of led lights on the market, such as row lights and ring lights that are always on. I hope I can help you.