...I read that I shouldn't miss out a grit ... when they begin to lose their edge what grit would you start with? 240 seems a little harsh.
You've put your finger on it. Just do what's necessary. No more.
When I can I favour steels over abrasives like emery/carborundum/oxide/stone/diamond. In any case I wouldn't bother with all those different grades even if I were starting on a blade blank. The basic Lansky Sharpening System only comes with three grades.
When I use an abrasive on a blade, which is rare, at most a coarse and a fine usually does it for me. The definitions of 'coarse' and 'fine' are often down to circumstances, i.e. whatever I happen to have handy at the time.
If the edge does a lot of work then it's going to wear and lose some of its sharpness. How fast it wears depends on all sorts of things:
the material of the edge
the degree of 'sharpness'
the edge profile - this is a big factor
the materials it cuts (and any foreign materials e.g. grit, nails...)
the amount of cutting it does per unit time
the way it's used to cut
the phase of the moon
Well I might not be too sure about that last one, but there are so many variables that you can't really make hard and fast rules. You have to take it as it comes. If it's doing the job reasonably easily it's probably sharp enough. If we're talking about a general purpose or 'bushy' type knife then a sharp edge will cut very nearly as well as a very very sharp edge will cut, but other things being equal the very very sharp edge will fade a lot faster. If you sharpen it when it doesn't need it (like if you have a sort of edge fetish) you're just wasting time and materials -- and shortening the blade's (long) life.
Obviously things are a bit different for something like carving works of art, where you'll want a razor-sharp edge almost all the time so you'll be sharpening often and stropping even more often.
