It really depends on the photo. A photo with large areas of one color, (lot’s of blue sky for example), will need a higher quality setting, because the gradients will quickly degrade to large blocky areas.
See the attached screenshot: The file-names (highlighted) show the quality settings used. You may need to view your screen at an angle to see the “chunky” sky on the left, the right is high enough quality that the gradient is pretty much smooth, and there aren’t many visible artifacts.
I think as a general rule of thumb, you can get by with a setting of 77 for Lightroom. It will probably halve the file-size, while to the casual observer at normal screen resolution, there is no discernible difference! Naturally, if you’re exporting for backup, or for prints, you’ll want high quality.
(GIMP uses a different jpeg algorithm, so the settings don’t necessarily carry over, I personally wouldn’t go below 75 with GIMP either. If the file size still isn’t small enough, you should reduce the resolution, instead of quality settings.) I’ve found that when you’re working on a project in GIMP, and you export a jpeg preview, you can set the quality to 95, only 5 less than High Quality, and it more than halves the file-size! See the illustration from my recent project below. (As you can see, I made quite a few revisions!) The two files that are circled were exported at 95 and 100 respectively! That is very useful when you’re just exporting a quick preview to show the current progress on a project.
