There isn’t really any way to know the exact time. Since the blocks will always take some amount of time to propagate across the network, and each proposer may be from anywhere. So you can only really time the duration between blocks that your own node receives. Maybe if you setup multiple nodes all across the world and average the time between blocks as they each receive them you could get a more accurate representation, but it’s not going to be far from 3.4 seconds at the time of writing.
Image taken from: Metrika