Boxing training has not changed very much in thousands of years.
For ancient Greek boxers and pankrationists, shadow boxing was a regular part of training, and they would often use hand weights.
These weights were not heavy. At most they might’ve been two or 3 pounds, and the goal was not to develop hand speed, but rather to develop endurance and proper movement mechanics.
Shadow boxing was a way to rehearse their fight skills and done for long periods of time it develops fighting endurance.
WARNING, if you have never shadowboxed with weights, you do not want to snap your punches, you’re not trying to punch as fast as possible, holding a weight in your hand has a different weight distribution than a boxing glove, it is very easy to hyperextend your elbow or injure your shoulder if you overcommit on your punches.
The focus is quality reps in duration, not trying to burn yourself out as quickly as possible.
How long the ancient Greeks practiced their shadow boxing is not known, obviously they didn’t have clocks like we do today.
We do know that because they fought so often (similar to Muay Thai fitness, having over 100-200+ fights was common), their training is described as consisting more of drills, shadowboxing and heavy bag work than hard sparring, in this way their shadowboxing could’ve been conceivably been done for 10,20, 30, maybe even 60 minutes at a time, or longer.
Lastly, the weights I’m using are 1.5lb weights from @eggweights