How much milk kefir can I give my cat?

I have a beautiful 22-yr-old cat in very good health, although mostly blind. He absolutely loves milk kefir (I make it at home, cow's milk). He eats every scrap of his regular food and begs for more, so I've been giving him a teaspoon of kefir from time to time, and he hasn't had any reactions to it. Not that I want to feed him a cup a day, but how much should I give him? Can I just give him as much as he wants and has no reaction to? He's being followed closely by the vet, and his bloodwork (every six months) is always excellent. Thank you so much!

Hello!  Has your cat always eaten every scrap of food and begged for more?  Is it his 'normal' or is this a gradual change?  I understand that a blood profile has been done in the last 6 months, but T4 (thyroid) and cancer for example, are not always tested for by these profiles (depending on your vets) and further, can arise at any time.  Increased hunger in the absence of weight gain is a clinical sign of concern and certainly worth a vet appointment.  Regarding Kefir, it is not a cats' natural diet or particularly recommended;  I hear that you are feeding it apparently without side effects, but cats have evolved to get their energy from meat so I would take your vets' recommend ation.