I have a crock pot recipe that tells me to cook this meal on low for 6-8 hours. WHICH IS IT?

that's a big time interval, at least i think so, so how do i know how long to cook it for if i've never made this before. also, it calls for sweet potato cut into bite size pieces-- can i just use the 'yams from a can', even though they're already softer? do i have to use actual sweet potato for the recipe to work?