As soon as I went full time fem, which was when I started presenting at work. That was ~20 months into my transition. Well, I'd used them a couple times before that when I was presenting fem in public, but I'd generally look for single occupancy or gender neutral bathrooms back then. It was a bit scary at first, but especially now that I have passing privilege, it's just normal and slightly affirming. I've luckily never had any issues in women's bathrooms, and I highly doubt I'd run into much issue in Seattle of all places, especially Capitol Hill lol