how long to try new food before switching?

[Bear & Pinot]

Bear has had some soft stools for a while now, nothing serious, but I thought it was from too many treats or my roommates sneaking him table food. After reading up online and thinking back, it probably started around the time that Canidae switched it's formula. So, I decided to switch him to new food, and also choose to make it a low-fat food. I'm making the transition from Canidae to Wellness Healthy Weight, and I'm on day three of slowly mixing them together. Well, his soft stools are now totally gross and runny! Should I keep going and hope that his stomach will settle once it gets used to the new food?

[rannmiller]

I hear it takes about 8 weeks for the food to get fully integrated into the dog's system, but I've also heard that digestive upset should stop after a week or two as well. So I'd say stick it out for another couple of weeks and see if it helps.