I agree with one of the very first posts... Except on very particular cases, the width of the guard ring is not important, the most important thing would be to have it there and having the correct depth and functionality.
What I mean with this is that, usually, rings are there just to collect some noisy or undesired signal components, but minor ones, that's why (I said) it's not important the width (you shouldn't drive a lot of current there).
One particular case (as I mentioned before) would be a substrate contacts ring around an ESD protection structure, in that case I would yes recommend having a wider enough ring.