I’ve been applying DIY 20% ascorbic acid solution on my face for a few weeks, and am seeing some benefits already. Photoaging and spots seem to revert a bit, and there is definitely some lightening (as expected [1]). According to some Reddit posts, it takes a few months until the improved collagen synthesis becomes noticeable.
Vitamin C serums are quite expensive because ascorbic acid oxidizes quickly, and other forms have to be used instead. But if you mix ascorbic acid with distilled water yourself, every few days, you get a very cheap and probably more potent alternative. I put it in a sprayer bottle, keep it in the fridge, and spray my face in the morning. I haven’t found studies on the ideal concentration so far, but consensus on Reddit is that anything between 10 and 20% is ideal for collagen synthesis. 20% is too harsh for some.