As many times as you want.
Note if your wanting to increase the power of the enchant on a weapon that came with one, and your going to improve the weapon, improve the weapon first, the enchant will almost always go up in power.
But, if you want to swap out the enchant on a piece of gear, you can do that as many times as you want.
Note 2 Master Weapons have special enchants and you probably don't want to replace those as nothing you can make will be better than those enchants in most cases.
Yep.
But if you get a dropped item in Blue, say an Armor Piece, that has a Magic Enchant on, as you upgrade the armor piece, the already applied enchant improves in power. So no need to waste Runes in this case.