I have my Fire-phoenix and Twilight phoenix. Thanks to a high lvl Timewarp and the Polar bear I can do the tourney in 1 day. When adding boosterbuildings late on Thursday one week I can still use them on Tuesday the next week, so the 3 of them eat 5 petfood/2 weeks.
Then the Red Pandamaster is something I keep fed (it is good for the next month at the moment, but with a 3-day feeding it means just over 2 per week on average) to boost the chests from the MA.
And I feed the Brown Bear once a month or so to create a crap-load of troops with the aid of a lot of time- and supply-instants.
Right now in chap 20 I'm low on seeds, so I feed the latest addition, the Raccoon for extra seeds.
So I use quite a bit of petfood; all in all I use about 8-9 a week when one adds up everything.
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I have found no shortage of petfood, but with the free diamonds from the Spire combined with the Sips of Clarity (also gained for free from the Spire and events) I can cycle through the MA regularly, so I see petfood appear pretty often. More then enough to keep my pets fed, all basically for free except time spent, keeping an eye on the MA and using the free resources available and the petfood generated in events and eventbuildings like the Savory Bites.
I know some people may not be able to do this as extensively, but it obviously is possible to get enough petfood using the free resources available in-game.