Don't get a fill unless/until you are eating so much food that you haven't lost any weight for several weeks straight.
Any doctor that insists that you need a fill based on a certain time schedule rather than how you feel and how you're losing weight is not a good band doctor, if you ask me.
If in doubt, I feel that it's better to err on the side of keeping your band too loose than to risk making it too tight. A too-loose band just means your weight loss might slow down for a while. A too-tight band can make it so you can't keep any food down, and that's obviously not a good thing at all.
Here's a list of things to consider in determining if you need a fill or not:
http://lapbandtransformation.com/Adjustments.htm