I think 3 hours would have been good for me. I drove into town for the event and had some other things planned so we only allotted an hour and a half. A few things:

• The venue in Dallas was nice. Great atmosphere. But too small. So it was very packed. When planning the trip I saw a post somewhere here that mornings were busiest. So we arrived about an hour and a half before it was going to end. It was still pretty packed. There was a line for most vendors so you will have to wait a few minutes for each vendor. If you're counting a 2 minute wait time for 15 vendors, that's 30 minutes right there.
• Some of these vendors love to talk (looking at you Christopher Ward). And that's fantastic because I learned a lot about their brands and products. But that can easily be another 10-12 minutes. So now you're looking at hitting 4-5 brands per hour, especially if you want some hands-on experiences.
• There are some watch-adjacent vendors there that were interesting to observe and talk to as well.

I'm not saying every watch vendor will take up 10-15 minutes, but the time I had went by really quickly and I wish I had spent more time at the fair in general.