I think they have misread your question. I believe they have answered as to how long a packet of critical care will be ok to use after opening. Rather than, how long can a rabbit be fed with critical care safely.

I may be wrong though, I would message them again

I’m sorry to hear your bunny isn’t well. I have never experienced such a long bout of stasis, but one of my buns once had to be syringe-fed CC for 3 weeks until it was found out that he had molar spurs.

Have you had your bunny’s teeth checked? They are difficult to see without anesthetizing the bunny. It’s very common that rabbits grow spurs on their inner teeth, and those spurs can hurt the tongue or cheeks, esp when the bunny tries to chew food.

If he has an impaction (hairball etc), hydration is extremely important. Sub q fluids are often needed, because the fluids/water you can feed your bunny might not be enough, plus there’s always a risk of the bun breathing wather into the lungs.

If Oxbow says CC is only ok to use for three weeks, it probably means they can’t guarantee the nutritional contents for longer than that. Not that it’s dangerous to give or that it necessarily has lost it’s nutritional contents. I’ve given CC from older packages than that, I keep CC in the freezer (as per recommendations). I think most of us here who have had sick bunnies have done that. There is another brand of recovery-food for buns, one that comes packed in small sachets http://www.supremepetfoods.com/recovery/ that I know people here have been using too.