What's the suggested minimum length of a woman's skirt?

It deends on a mixture of the situation, what your wearing with it and the look of your legs.
I'd suggest for most business and formal situations its best to play safe with an inch or two above the knee minimum, any shorter is tarty and looks like you're purposely trying to utilise your sex appeal rather than your talents for advancement - not that there's necessarily anything wrong with that, but there's no need to make it obvious- if you want to look sexy AND proffesional, go for a knee length pencil skirt.
For other situations, your first priority is to check that you won't get arrested when bending over. Check your skirt lengths from the back as well as the front to check that they preserve your modesty. After that you need to think carefully about whether your legs can take it. If you have great legs, by all means show them off in a mini and knock 'em dead, if you have slim, pretty calves but chunky thighs, you need to stop at the knee, if your legs aren't your best feature stick with trousers or long skirts and flash some cleavage instead when you want to be sexy.
Also, pasty, stubbly and "corn beef" legs need tights or stockings under a mini, however shapely they are. My personal opinion is that other than for a big night on the town, short skirts are best worn with thick tights and flat-ish knee boots or ballet slippers to avoid looking like a hooker, which tends to happen if you insist on towering heels. There's also a rule worth remembering that if you're revealing the bottom half you should be demure on the top half (and vice versa) so if you're showing your legs, wear a high necked top, preferably with long sleeves, to avoid attracting the wrong kind of attention.

Depends on what the situation is. For most business/professional occasions (work, conferences, meetings, interviews), neither men nor women should dress provocatively. Most would agree that knee-length skirts are acceptable, though I notice that women's choice of skirt or slacks to be worm with a professional outfit is about 40:60 these days.