I am not confident that there is a good answer to this question without a lot more detail. It is true that by watching the income and sinks of the currency supply you can track overall currency in circulation, but that is not strictly the same as inflation. They may be (and usually are) related, but there's a lot of other factors that go into it. All you can really track that way is total net worth of all participants.
You could mean in a sense of buying power. (Ie: increase in the overall cost of most goods/services):
In this sense, it will depend strongly on how the money is being generated and sunk. Do all users want the same kinds of things? For example, in WoW, you wouldn't see a level 1 guy looking for the same weapons as a level 50 guy. In a situation like that, if the level 50 (presumably) is getting a bigger chunk of the increased money supply, then his items will inflate much faster than the items that the level 1 guy is interested in.
It also depends on how equitably the increased money is distributed. If only relatively few characters have a ton of money, prices will likely be very unstable. Everyone may want to sell items for very high prices banking on the one dude with a ton of money strolling through and buying it, not caring that it takes a while to sell. Or it may be the case that if those "big fish" aren't biting, prices remain the same for a long time because even though the currency supply is bigger, the average Joe doesn't get any of it.
Also how often are items sold back and forth? Is everyone a solid "producer" or are there people whose net worth is likely to just keep going up and others who will remain at a fairly steady net worth?
To hazard one practical useful answer: You want a little bit of inflation, otherwise there is no impetus to spend your money and the economy stagnates. This is especially true if there is deflation. However, you do not want so much inflation that money earned today evaporates before it can be spent. Let's assume everyone's income increases at roughly the same percentage rate.
The exact rate would have to be dependent on how fast you expect people to spend their money. Are they making daily purchases or are they likely to save up over a long time to make a few big purchases? With a mixture of both, I would probably go with a figure close to the real world value of 1-2% a year.