I think it’s better to determine the point at which you’re personally no longer ‘happy’ with the return on your investment. That number is different for everyone, so it’s wise to know about how much you get at any given moment.
Once you have all the Commons, then the vast majority of the cards you get in packs are turning to dust. This usually happens somewhere between 20 to 30 packs. Once you have all the Rares as well (somewhere around 70 to 80 packs) then your median pack result is going to be 40 dust.