Hi Matt,
This describes duty cycle some...
http://www.zena.net/htdocs/FAQ/dutycycle.shtml
[edit..I'm not pushing these welders, because I never heard of them before. I included the link because the definition was pretty good at the top of their page.]
Here's what AWS A3.0-94 Standard Welding Terms and Definitions says...
"Duty Cycle=The percentage of of time during an arbitrary test period that a power source or it's accessories can be operated at rated output without overheating."
Good question,
John Wright