How long does it take for a dog's stomach to adjust to new food?

For most dogs, the honest answer is about a week to ten days, provided you make the change gradually rather than all at once. The usual method is to mix a little of the new food into the old, then shift the ratio across those days: roughly a quarter new for a couple of days, then half, then three-quarters, then the lot. That slow handover gives the gut bacteria time to catch up with what's arriving in the bowl.

Thing is, dogs aren't all the same. A young dog with a cast-iron stomach might sail through in a few days, while a sensitive soul, an anxious rescue, or a dog with a history of tummy trouble can take two to three weeks to fully settle. Even once the transition's done, it's not unusual for stools to stay a bit unpredictable for a week or so while everything beds in.

The thing to watch is your dog's back end, basically. Firm, settled stools and a keen appetite mean things are going well. The odd soft stool early on is normal. Ongoing vomiting, runny stools for more than a couple of days, or a dog that's off their food and flat is your cue to slow the change right down or check in with your vet. Rich or high-fat foods tend to need a gentler, slower run-in than lean ones.