I have an 8 year old cairn terrier who has just been diagnosed with immune mediated polyarthritis after almost six months of tests!
She is now on steroids and as the blerb says is wanting to eat and drink alot, my question to you all is how much is too much water?

I hate to generalize, but as much as she wants. When a dog is urinating a lot, whether due to drugs or disease, they need to replace what's being eliminated. Otherwise they can quickly become dehydrated.

Food, on the other hand, needs to be monitored and controlled. Steroids will cause weight gain anyway, so you have to adjust the diet to keep them from turning into watermelons on legs.

[Yogi]

in addition, steroids do increase intake of water and thus need to urinate more often. As vetgroomer put it, this should not be discouraged.

well it is really hard to measure that, if i where you try to observe your dog and know how much is too much water for him

I have a pitbull that hurt his back and has been on steroids. The vet told me that he would drink and therefor pee more. I was worried he was drinking to much because it seemed like he was drinking GALLONS a day. The vet told me not to worry and let him drink all he wanted AS LONG AS he was still going pee. He said not to worry unless he stops going pee pee. Now he is on the tail end of the steroid cycle and his drinking has gotten back to normal.

If your dog is taking a Steroid, it can really make her very thirsty and of course if she drinks a lot she will urinate a lot. Do not limit the intake of water from your dog, because it'll lead to dehydration. Always make sure your dog has plenty of water during the day.