I create a new character when I get the idea for them - sometimes a power combo, sometimes a concept. I'm finding concept is most important to me, I've created and then let sit idle a number of toons only to rejigger the look, name and costume and then suddenly zoom them up to 50. If they bore me, or have a thin a thin concept ("Rabbitattatt, a rabbity-looking toon with a machine gun, genius!")- or yeah, play badly for some reason - I drop them, but rarely strip and delete them. I generally try to not repeat power sets or at least not too close to one another. Started an abandoned like 3 Kins by now, it was just a boring set to play!
I've run one of about every AT up to 50 except for VEATs and squids (never even started a squid, seems complex and I haven't been enticed yet). I have about a page or page and a half of 50s. Realizing now some of them were fun at the time but I have no desire to go back and work on their Incarnate stuff. Realistically, I should strip and delete these guys, I figure about half of them would be on the chopping block.
I have a fair number of created toons that never got past say 15th, as I found I lost interest or just didn't have a direction to go with them; for instance, I made Symphony Dom when that was new, I think she got to 25th? But no real build in mind, MIDs hates updating for me... so she got left behind.