It is different for NHS and private patients.
For NHS, you get first fill about 6-8 weeks after the op. They normally put a very small amount in to begin with to get you used to the feel of some restriction. Saying that, often or not with first fill there is hardly any at all. The second fill is usually booked for 3 months from that date, and that fill is often bigger. There is no prescibed amounts of fill, as each person is different. It is done either under xray or with a cold water swallow test. The third fill is booked for 6 months later but with my surgeon he always says, if you are feeling that you can eat too much or too much of marker foods like bread, rice and pasta to call for an earlier date.
I had to do this as I was not loosing and being able to eat too much.
The third fill has taken my band to about half way and I have very reasonable restriction. The band naturally aspirates or looses a little saline, so although next fill is not due now till October, if I start to be able to eat too much I will call for an earlier fill.
The fill itself is almost painless. The needle is very small and as it does not go intramuscular and just into the port it is less painful than a standard injection or blood test.