No one has answered Randy's first question yet so another Randy will have to do the dirty deed!!! [xx(]
The answer is as ofter as necessary!!! Not exactly what you wanted to hear.....right?? It is, however, the correct answer. If you are making pens and using a soft wood, you may be able to make quite a few of them before your tools need to be resharpened. If you are making a larger item out of a hard or abrasive wood, you may need to sharpen your tools several times during a single project. After you have been turning for a while, you will develop a feel and know when your tools are not doing their best.....that is the time to sharpen them!!! Another consideration is the quality of your tools. Some are made out of better steel than others and will hold an edge longer than others.
Of course, another question is what do you mean by sharpening??? As an example, a chef at a big restaurant may not put his good cutlery to the wheel but once a month. On the other hand he week probably hit that blade with his "steel" several times a night.
As was mentioned earlier, once your bevel is established on the wheel, you shouldn't need to go back to it very often.....unless you are a very high volume turner or unless you have a accident(hidden nail for instance) and screw up an edge. If you are regularly touching up your blade with fine sandpaper, a natural or diamond stone or one of the honing materials, you could go a long time before ever putting your tools back on a grinding wheel. Sharpening tools is not the easiest thing in the world to learn and it's a lot easier to pick up if you have a good in-person instructor than learning from a book; but once you learn the principles of sharpening, you can pretty much sharpen anything that will take an edge. If you don't have someone available to help you, probably you should look around for a good video. I can't suggest one, personally; but maybe someone here can offer a suggestion.