For an amateur telescope, what is the general limit for magnification before image quality significantly degrades?

About 50 times the diameter of its aperture in inches

While magnification is easily calculated as the ratio between the telescope's focal length and the eyepiece's focal length, excessive magnification severely compromises image quality because there simply isn't enough collected light available to adequately illuminate the enlarged view. For practical amateur use, there is a general ceiling imposed by atmospheric turbulence and insufficient light gathering for the size of the aperture. This theoretical practical limit is usually stated as being no more than approximately 50 times the aperture size when measured in inches. For example, a 6-inch telescope should ideally not be pushed far beyond 300x magnification if the observer expects to see a usable, non-dim and non-blurry image.