I noted that when visiting my sister down in the Bay Area, I had to steep for quite a bit less time before the bitter tannins would start creeping in. Like 1.5-2 mins tops for cheap PG Tips. But that same tea up north could sit for 3-4 minutes before the bitter tannins would creep in.

It was a marked difference so there are obviously some confounding factors. I suspect the water chemistry matters a fair amount.