vegging is when your plants are growing bigger like stems, leaves and roots, and its really up to the grower on what the lighting should be for vegging, some people chose 18/6, some choose to have lights on 24/7, either way the plant will stay in veg. My personal oppionion would be 18/6 as soon as the sprouts come up, I remember hearing somewheres thats roots grow during the dark periods.
As far as switching to 12/12 again its really up to the grower, the longer you grow it in veg the bigger it will be, some people will veg for 6 months before they switch to 12/12, then others will switch to 12/12 after the plant is 6 inches tall. All in all, the longer you leave it in veg, the bigger the plant gets, and the bigger yeild you will have.