There are websites that track it. Check them. Usually early October is good time to visit Portland
Now
I live here and some of the leaves are starting to turn. I’d maybe wait 2 weeks.
Planning to visit next week. Hoping to see a great fall foliage! Any recommendation?
Drive up highway 30 along the Columbia River, from Portland to Astoria. Stay overnight. Then drive down 101 until you hit highway 22 (or you can go a little further to Lincoln City if you want, then backtrack to the highway 18 interchange). Then come back to Portland via highway 18. You'll go through the Coast Range, then through Oregon wine country, so stop along the way for some for some little hikes in the mountains and wine tasting in the McMinnville/Dundee/Newberg area. The Travel Oregon website is also a pretty good resource.
Now through mid november
In the fall.