Two and a half years. Their first releases were about two weeks before Christmas in 2008.
I do remember a lot of teeth-gnashing from Criterion DVD collectors before/when they dove into HD, and since it took them a while to fully adopt Blu-ray (they were still putting out DVD-only releases nearly two full years afterwards), there were also complaints about certain titles bypassing HD. And while it's definitely not the same scenario, there was significant blowback when Criterion switched to single-SKU combo releases in 2013, with all sorts of complaints about Criterion forcing DVDs