Theme Designing

Didnt know exactly where to post this but I recently been trying to learn how to design themes. I know HTML5 CSS and all that and have a pretty good under standing of PHP, cant develop PHP by scratch yet but I can tell what code is trying to do or manipulate it to work.

Anyway, my question is this, for those of you that develop and design themes, do most of you do it as a team or all by yourself? I am trying to do it by myself, but if I find out that almost no one creates them by there themselves without some help, then I might as well find a partner. If not, Ill continue to try and do them myself.

Most of this is pertaining to the commercial themes that costs 40+ etc.

Also if anyone can help out with that question, I might as well add, about how long does it take most of you to create them(ballpark range). Like weeks, months, etc?

Thanks.