A counter overhang can be any size you want it to be.David Sacks/Lifesize/Getty Images
Countertops can be installed in a variety of ways, and there are numerous patterns and materials from which to choose. As far as any overhang goes, it's a matter of personal preference. You may want an ample overhang on an island so that family members can pull up a stool and have breakfast, or you may not want any overhang at all if your kitchen doesn't have much space. There's really no standard size.
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You may want your counters and cabinets to be flush with each other. Although this is not a common choice, it works for butcher block, because it can create the illusion of the cabinets extending through the countertops.
Most kitchen counters are flush with the doors of the cabinets, which generally extend beyond the cabinet frame. This is simply a matter of aesthetics, as the look seems thought-out.
While you can extend the overhang as far as you want, extending it too far beyond the cabinet base will make it more difficult to get into the cabinets. This can pose a safety, as well as an accessibility issue. Going more than an inch beyond the cabinet door is not recommended unless you don't have any cabinets under the overhang.
You will need to brace any counters that extend a fair amount beyond the cabinets. A braced overhang can extend as much as 18 to 24 inches beyond the cabinets.