It's strange to being using Hacker news as a nutrition advice, but this is something that I have been reading research on Pubmed for years. (Actually going back to the grateful med....).
Approximately 50-60% of brain weight is made up of lipids, of which about 35% consists of omega-3 polyunsaturated fatty acids (PUFAs). Of the omega-3 PUFAs present in the brain, DHA accounts for more than 40% of total omega-3 PUFAs in neuronal tissue, especially in gray matter.
Now I know this is slightly different than the OP, but I consider the primary reason for incorporating Omega3 in your diet is specifically to give your brain the basic building blocks that it needs to create the right structure. And the issue is you have to eat Omega-3. It's not something that you can manufacture: you can take a shorter chain and potentially increase the length of the carbon to get it all the way up to an EPA or DHA, but even this falls off as you age. So I don't consider taking flaxseed oil a good idea.
Without spending a long time on this, Long term supplementation I believe we can come up with an extremely strong line of evidence that suggests it helps neurological and emotional health. Now while there may be other benefits, this one is so massive and so clear to me, this is the reason that you want to make sure that you get this basic building block.
What is clear the western diet for the most part is severely lacking in in Omega-3 (or often called N3 PUFA in research), therefore if you're not thoughtfully measuring what you take in, you very well possibly could be running low especially considering that there is a significant Sigma in terms of what people need to have a healthy brain.
So now let's talk about the food supply. The problem is virtually every fish in the world has mercury higher than what you want. The only fish that looks like it has an acceptable level of mercury is salmon in certain geographies such as Alaska. My guess is you're actually eating some type of farm raised salmon, And I would have concerns unless you exactly understand the sourcing.
Without getting into the details, It is phenomenally difficult to come up with clear guidelines for biological humans that have wild diversity in terms of genetics and how your body processes food stuff. If anybody tells you they absolutely know the right dosage for you, immediately you should call BS. The data is way too far ranging to suggest a simple and clear case of what any individual needs.
Now keeping that in mind, I would suggest that most people probably need around one gram worth of EPA slash DHA in their diet basically forever. Depending upon your genetics maybe you could go down to half of this.
This is not a number you should guess at, and with things such as perplexity, You can find out how much EPA and DHA you are taking in in your salmon. You should be able to find out the number of ounces and talk to the cook about what they are serving. And 100 grams of salmon depending upon if you're eating clean salmon which has lower levels of EPA/DHA or farming salmon which has more oil but normally is dirtier, you should see somewhere between 1 to 2G.