I guess it depends, I just monitor and if they're not dying fast enough or if there's a point where the character dies more than twice on an elite, I know its time for an upgrade. And it'll depend on the last upgrade whether or not I need a new one soon. Right around high 20's, I just buy really cheap leveling gear. We're talking all about some mainstat/vit and +exp stuff. nothing else. boots have movement speed that's about it. Helm also has to have socket. You can get these for like 1500 gold.. costs nothing. I would recommend you just keep these so you dont have to hit up the AH if you relevel a lot. (multiple accounts).
Then around high 30's I regear only one or two things. Like my rings and armor were legit so they didn't switch out. Only switching things out if I absolutely need to. Usually around this time, self found gear will be fine.
Around 45 is when I hit up the AH cause right around this time you need higher DPS. I get new weapons with -15 or whatever level requirements. Upgrade armor if I have to but usually I don't. The DPS increase in my weapons alone will suffice. I always go 2 handed cause they die before they can even get to me.
Then around 52 or so, I hit up the AH again cause things get harder and die less quickly. You can get 800 dps weapons with lots of mainstats for 150gold. cost nothing.
Then the full regearing comes when I hit late 50's or whenever I get into inferno cause that's when I need the all resists, etc.