face recognition screen lights up goes dark in a few seconds

That is not the face recognition screen. You are talking about doing something on the iPhone screen. If you do not have Attention Awareness activated and are looking at the screen and are not actively doing something like typing, then the screen will dim and auto-lock based on your settings in Settings>Display & Brightness>Auto-Lock. If you wish for it to say bright longer, you need to increase the time period for Auto-Lock. I'm sorry I misunderstood your question since you said lock screen and a few seconds. That made it sound like you were trying to do something with the unlock screen. It was not very clear.

What are you trying to do that you need it on longer than that?

I can understand that. Glad I could help.