It depends on your approach. Several things:

First, I love buying ONE booster box to rip open (just to flip cards) asap. Ebay's sportsandmore usually has early boxes for a steal. What I don't want, I sell, and keep what I think I want or will go up in value, but most will not go up, so selling is a priority as I know prices will, on average, go down within a few weeks.

Usually, every set will have a chase mythic. Rekindling Phoenix is for Rivals of Ixalan. Carnage Tyrant WAS for Ixalan (although it's rebounding a bit). Whatever Mythic I think it will be, I spec purchase multiple copies. I'll also spec on lands, as lands do a great job at holding or gaining value (ex. Fields of Ruin, Unclaimed Territory, and Scavenger Grounds).

Attending prerelease drafts are also important; they give you great bang for your buck and access to early cards you can flip for great money, since not a lot of supply has entered the market yet. Furthermore, it gives you on-site knowledge to brainstorm what will be the next meta. Prerelease is when I discovered Rekindling Phoenix is broken as hell, so I spec bought 8 copies at $13. I wish I had bought more!

Knowing all this from my selling perspective, you have a better idea then when to buy. I'd say one month for singles, but until then, entertain yourself with your own booster box and attend prereleases.