Mr. Bartley's response, quoting factory literature, is of course completely accurate. However, this manufacturing tolerance is for new shutters.

My experience (rather than quotable documentation) is that electronic shutters (as in new 35mm cameras) are right on the money.

Mechanical leaf shutters, as in view cameras and Hasselblads, are activated by more than one set of springs for various speeds. It is not possible to time the length of a one second exposure and translate that error to the 1/125th second setting.

These shutters are almost never too fast. With time, fridgid environment and prolonged inactivity, they tend to slow down. One of my duties as a studio manager in Hollywood was to get out all the lenses every Friday afternoon and repeatedly work their shutters.

If you can afford it, having your shutters serviced and calibrated by a professional is a good idea. A cheaper solution might be a simple digital shutter speed tester from Calumet.

If you have only one shutter, base your film exposure tests upon whatever it is giving you. It doesn't really matter what the actual speed is, as long as it is consistent each time.

Just remember that buying another lens will require more film testing.