Bleach will not necessarily make the fabric a lighter shade of the same color. It may result in an entirely different color or it may have no effect at all. It all depends on what specific dye was used to color the fabric initially. For example, some dark blues bleach to red, some reds to white, some blacks to brown.
The vinegar (hydrogen peroxide or anti-chlor are better options) is a stop-bath and is absolutely necessary. It does not 'set' the color change, but is used to stop the bleaching action so the fabric is not totally destroyed (meaning eaten) by the bleach. Also, be sure to wash all discharged fabric in your washing machine, with detergent, when your session is done. Read up on discharge dyeing for more details.
To answer your specific questions, for what you're looking to do, I'd use a 1:10 ratio of bleach to water (1 cup bleach to 2½ quarts water) and just soak the fabric until it's faded to your liking. Each dye will react differently so there's no set time to go by. I haven't done an overall lightening of printed fabric, but I frequently create patterns on solids with bleach using various marking methods. It's great fun. Enjoy it!
