Have been holding off buying our tickets to Paris from Boston hoping they may come down.How much longer should I put off buying them for September?

Base it on your comfort level. If you had said when to buy for July, I would have said February/March. For September, a few years ago (although this is from LAX to CDG) I found that by waiting until almost the last minute (mid-July)I got an embarrassingly cheap flight on Air France, passing up my mom and a friend who bought theirs months before.

For September, I'm a firm believer that the airlines start to panic around mid-July, and lower their prices to get a fuller plane.

I should have qualified the above -- that's for steerage (economy). If you're planning on going business I'd start checking twice a week, early Tuesday morning and late Thursday evening (as an example) to get an idea of pricing, and once you find a price you can live with, take it.

I had to do that this year with Air France/KLM (actually found the KLM website cheaper and easier to navigate that Air France U.S. even though they're partners). And I'm not going until early October, but I wanted to go Biz class AND I wanted to have a good selection of seating, along with the best price, so I had a lot of priorities to fulfill that probably wouldn't have worked had I waited, and had I gone economy.

Just remember the Surfergirl didn't put any money on the table. So that advice comes with no risk. Cheap fares occur when there are surplus seats to sell. When or if that happens, no one knows. With the recent mergers and consolidation there is much less capacity than a few years ago and, in general, air fares are higher. The longer you wait the less choice you will have. So you have to decided if you are willing to take risks associated with waiting till July. September is still in the peak time.

Personally, I would be nervous waiting until July but that is me. We are also traveling in September and the fares certainly are high! I followed the fares for a while and then just resigned to them and purchased as my dates aren't flexible.

I am seeing fares little over 1000 dollars from Boston - Paris by way of Dublin.
A gamble as to whether to wait or not.

It is a gamble, isn't it?

It really is a comfort level. I tend to buy earlier rather than later, since gambling makes me nervous. You wouldn't think so based on my first comment, but I actually decided to take that trip last minute, so was surprised by the fare reduction. In my real world I tend to book early.