what is a long skirt?

I would say ankle length, but that's just my opinion. Definitions of skirt lengths that I've heard include mini (above the knee), knee-length (what it sounds like), midi (mid calf) and long (ankle). I think you could probably figure out what is required by thinking about where you're going to be wearing the skirt. Good luck!

In what context? If you are referring to dress codes to visit churches, I think mid-calf would be safe; I've even seen nuns wearing skirts that hit just below the knee. If you are referring to formal attire for the opera or something...???

If it's to visit churches in Italy, it needs to cover your knee - so slightly longer than knee length. I've only seen it enforced in some Italian churches (St. Marks in Venice and the Vatican in Rome come to mind) but dressing conservatively when visiting a church is always appreciated. Jeans and pants are also okay. The point is to have your knees, shoulders (and midriff) covered in some way more than being dressed "up". The only other potential problem I can think of would be a vulgar t-shirt design.

It really depends on where you are going (country and specific place). Yes in a religious building you should dress respectfully, some places will not let you enter otherwise. So long/calf length trousers/skirt over the knee is best. Be sure to take a wrap or scarf which you can put over your shoulders, if you are wearing a vest top. If you are going to a Muslim country e.g. parts of Bosnia in small towns/villages you should again follow the same concept, as you more likely to draw a bit more attention. But out and about in a larger town especially in summer people are in shorts/t-shirts and very liberal. Sometimes places have garments you can put on e.g. I've entered Mosques where I have had to cover my head, so they are prepared for tourists anyway, so don't worry too much about the dress-code.