The biggest mistake I see when people make risotto is they add way too much liquid. If you add too much liquid most of it evaporates than gets absorbed by the grain.
While risotto is a "cook it until it's done" recipe as @Joe stated, there are some "rules":
- 1/2 cup of stock per session and never stop stirring
- At about the 20 minute mark, with Arborio, it should be close to done, I usually start tasting at about the 15-18 minute mark
- The right heat, as with most cooking, is paramount to a good outcome, you want a slow and steady simmer on when liquid is the pan
- It is time to add more liquid when there is none in the sautee pan
Without more details as type of rice, process, or whether you are strictly adhering to a recipe, it is a bit difficult to give you the best advice.