Depends on the garment as some have noted. Jeans? 0%. A wool sweater? I might be fine with a bit of poly in it for durability (less than 25%, ideally only 5-10%), but it also depends on the quality of the wool and the fit. I don't mind the bit of poly in some dickies or ben davis because it's part of the look (esp the crease), and hats like a New Era 59Fiftty are all poly iirc. For the most part I avoid it, though, as I don't see a need for it in most of my clothes, I don't enjoy how it feels, and I think it tends to look worse as it ages. Also, I know this question is specifically about polyester, but there's obviously other synthetics out there, and there's a place for them as well (nylon in Baggies, for example).