What quality to choose when converting to JPG?
When you export a picture to JPG you can normally choose its quality in a 1-100 scale. I like to keep a good quality but it wouldn't make sense to keep a JPG that will be almost as big as the original RAW, so is there any guideline to select a particular JPG quality for particular uses? I am mostly interested in Internet sharing and ordering prints online.
This depends on the software you're using (e.g., the Lightroom recommendations in jrista's answer aren't the same in other apps); what is your workflow?
To be frank, it is entirely anecdotal that a JPEG image should be exported at a certain compression level all the time. The amount of JPEG compression should really depend on the usage purpose for the JPEG, and the contents of the JPEG.
The quality level one should choose when exporting an image to JPEG is highly dependent upon the kind of detail contained within the image. An image of a smooth blue sky or a sunset sky with large areas of orange gradient should probably use a high quality setting, 90-100. An image that contains nothing but complex detail could probably get away with a quality setting of 50-60, possibly even lower. There is no single "best" JPEG compression setting, and depending on the type and complexity of detail (or lack of complexity and detail), you may find yourself using 40-60, 70-80, or 90-100 as appropriate for the photo(s) you are exporting.
An excellent visual resource for how JPEG compression affects IQ can be found here:
This site demonstrates JPEG compression from the lowest to highest settings in discrete ranges for a series of sample images of differing content. You can clearly see, by observing each image at each compression level, why a higher setting such as 90-100 may be required in some cases, and a lower setting such as 40-60 is entirely acceptable for others.
As a side note, if you are saving a JPEG for any kind of print purpose, or for viewing on-screen at a large size (i.e. as a wallpaper for a 30" 2560x1600 screen), there is no reason not to use the best quality setting available. When saving for web, compress as much as you can without introducing visible compression artifacts. Different classes of images will regularly fall into certain JPEG compression levels (the site linked above can help learning what fall where)...so it can quickly become second nature to know what compression level to use when saving various images for the web.