Hi,
It very much depends on the weather as to how much she should drink. UP to 1 litre could be normal in hot climate.
I have a 19 month old daugher, and between breast milk and water I think she does drink close to 1 litre.
However, it is a very simple matter to get her sugar levels checked and you should definitely raise it with her pediatrician if you are concerned.
I don't have a clue how much children that age normally drink (perhaps some other moms will answer), but if you suspect she is drinking too much you should have her pediatrician check her out. If you suspect diabetes, it would more likely be type 1 than type 2, btw.