What is the standard ratio of sugar to cream in Crème Chantilly?
You can be quite free with your sugar amount, depending on what you want to achieve. I would say that as a rule of thumb, you don't want to create too great a contrast between your sauce and your other components. So, if you are using it for dipping fruit, you can use less sugar than usual, else the fruit will be perceived as too sour. For a rich torte, you can go up with the sugar. 

As for the normal range, it can start at zero, but this is very unusual for sweet applications. The upper bound is given by foaming: you can't whip cream well if it has less than 30% fat. (This number is for pure raw cream, and varies even there, depending on factors like cow breed. It can be slightly higher or lower depending on cream treatment - pasteurization etc. - and the amount of stabilizers usually added to whipping cream). So, if you start with a cream which has 35% fat, you shouldn't add more than 16.7 g sugar per 100 g cream (40 g sugar per 250 ml). Else your texture will suffer. 

You also ask for a standard amount. This will vary regionally. The Professional Chef recommends 57 g of sugar per 454 g (a pound) of cream, which translates to 12.5 g per 100 g, or 30 g per 250 ml. This is an American book, and I have found that using 2/3 to 1/2 of the recommended amount in most American recipes results in good-tasting sweets, whereas the original amount often produces oversweetened dishes. This would be my guideline for a standard amount too. You can adjust it to your own taste. 

On a side note, The Professional Chef insists that it whips best if the sugar is sprinkled in after you have achieved soft peaks. I haven't done it this way, but it is worth a try.