NEW YORK (TheStreet) -- Much to the experts' surprise, the United States may be on the verge of another refinancing boom. The average rate on the 30-year fixed mortgage has fallen to 3.9%, down from 4.5% last spring.

As The New York Times put it in its column "The Upshot," anyone with a mortgage obtained before mid-2011 or between late 2013 and early 2014 might well save money by refinancing now.

But the chance to replace a mortgage taken out just 12 months ago highlights a dilemma that many borrowers fail to consider. If these refinancing opportunities come along fairly often, should you do it as soon as it will pay, or wait until you think rates have hit rock bottom?

After all, refinancing is not free. Every time you do it you will pay thousands in fees. Refinancing makes sense only if you will have the loan long enough for the lower monthly payment to offset the fees. Accumulating fees from multiple rounds of refinancing means you have to stick around longer.

If you had refinanced a year ago and figured it would take three years to break even, you still have two years to go. A refinance done today might break even in a couple of years, adding more time to the break-even period. Stay in the house for many years and the refinancing probably makes sense. If you might leave in three or four years, maybe not.

Ideally you would refinance just once. Unless, of course, the rate falls to a new low that just makes your mouth water.

Timing the refinancing, then, boils down to a bet on what interest rates will do. If you think they'll fall another half-percentage point or more in the next year or so, it could pay to wait until they do.

Rates fell to an historic low of about 3.3% in November 2012. If that happened again, it might pay to refinance a loan taken out today at 3.8% or 3.9%. But that was an historic low. Over the past 40 years, rates have rarely been below 5% and have spent much of the time above 7%.

Today's low rates are largely the result of international economic uncertainty, which drives investors to the safety of U.S. bonds. High demand pushes bond prices up, driving prevailing interest rates down. Also, tumbling oil prices have reduced worries about inflation, allowing investors to settle for lower bond yields.

Slow economic growth around the world isn't likely to change overnight, and there's no reason to think inflation will pick up suddenly, so current conditions could persist for many months. A scare from an economic jolt, terrorist attack, virus outbreak or other trouble could send mortgage rates down further. But, obviously, that's unpredictable.

For the ordinary homeowner, the best approach is think about how much would be enough in savings to make a meaningful difference in the household's finances. Use the Refinance Breakeven calculator to see what you could save if you refinanced today. Then redo the calculation assuming a rate that's 0.25% to 0.50% lower.

You may find that waiting for rates to fall further does not offer enough benefit to offset the risk that, if rates go up instead, you'd miss the opportunity you have today.

Read More: 4 Things Homebuyers Have to Think About Before Signing the Papers