Quick one - how long for seo changes to take effect?

depends what you changed. title and on-page edits, two to six weeks after google recrawls. new pages or rewritten content, three to six months before it settles. links, a couple of months. technical fixes like speed or indexing, you see it in search console within weeks. with 300 products don't change everything at once. pick the category pages that already get impressions in search console, fix those, request indexing on each one, and watch them. if those move in a month, roll the same changes out.