I think it would depend greatly on the screen itself. Size, general brightness, number of backlights, edge lit, TN/IPS/VA, etc.
But I can say that my preference for running a laptop at zero brightness at work means my battery tends to last the life of the laptop. Not so much my co-workers. Maybe get an extra hour out of it, but that depends on the workload the laptop is doing (Which when I am on battery usually just means sitting through meetings, VOIP/Screen Sharing and maybe using a browser or Excel)
Depending on your laptop size and such .. it can lets say with a normal 3 hour battery life at 100% battery for some of the laptops we have at work.. IF you run it at like 50-60% brightness it esily adds 30 to 60 minutes to the life. it really depends on the size of the monitor and the battery size. like we have some laptops that should last 7 hours between charges or longer but in reality 3 to 4 hours is normal when you have the brightness at 90% or so..
Assuming the same size screen; a screen that has 400 nits of brightness at 50% brightness would technically consume the same amount of power as a screen with 200 nits of brightness at 100%. Adjusting brightness can give you extra minutes of battery life, but that's about it.