Q.How long does it take for dormant plants to start growing – i.e. Yuccas
Dormant plants…how long do they take to start growing? Just bought yucca and other dormant plants for first time.

I'm not familiar with any plants that would be dormant in the middle of summer. I suppose someone could force dormancy for shipping, but after being moved to a hospitable environment, the should start growing almost immediately. This is an article that will tell you more about dormancy: https://www.gardeningknowhow.com/garden-how-to/info/plant-dormancy.htm
P.S. I would be suspicious of anyone who's selling dormant plants at this time of year. If they grow and all is well, I'd love to hear about it.