Eight weeks of azelaic acid and I cannot tell if anything changed — how do you decide to keep or drop a product?

Eight weeks is genuinely early for what you are asking of it. Surface turnover in an adult is usually described in the four to six week range and slows with age, so you are barely through one and a half cycles, and redness and pigment are the slowest things to shift. For anything tone related I do not form an opinion before twelve weeks and I do not make a final call before sixteen. Texture and breakouts move faster, so if you were treating those I would agree eight weeks is a fair judging point.

Photos, but with rules, or they are useless. Same corner of the same room, curtains open, no overhead light on, phone propped at the same distance, same time of day, once a week, no expression. I keep them in a dated album and only compare week one to week eight rather than looking at consecutive weeks, because week to week your skin is mostly telling you about sleep and salt. When I started doing this properly I dropped two products inside a season that I would otherwise still be buying.

Half face test if the thing is symmetrical. I did it with a niacinamide serum for six weeks, could not tell the two sides apart in photos, and stopped buying it. It feels ridiculous for the first week and then it is the only test I trust.

Slightly against the grain: if you cannot tell after eight weeks of daily use with no irritation, that is a result, not an absence of one. Something that is quietly working usually shows up as fewer bad days rather than a transformation, and you would probably have noticed fewer bad days. At 24 a tube every six weeks, the honest question is whether that money buys more elsewhere, and for redness specifically I would rather put it into a better sunscreen habit.

Twelve weeks, weekly photos in fixed light, one variable at a time. If it fails that, it goes. Nothing gets a third tube on faith.