when you water, you should always water till you see a pretty good amount of water running out of the bottom of the pots.. about 20% or so of the total amount you watered with is a good rule of thumb..
as far as how long to veg them till they're ready for flowering, that depends on several things.. the first one that affects most people is how much head room you have for the grow.. if you don't have much, it's better to flower them sooner rather than later as they will at least double or possibly triple in size from the time you put them in flower.. this means a plant that is 12 inches when you flip the lights can easily end up at around 24 / 36 inches, depending on the strain of course.. indcia dominant strains tend to stress less, and sativas will stretch more..
i don't have any head room problems, so i always like to let my plants veg till i see pre flowers.. there are plenty of pix around of what preflowers look like... preflowers indicate that the plant is sexually mature and is ready to be flowered at anytime.. of course you can flower sooner, but it's like a 13 year old girl having a baby vs a 22 year old having one..
lastly, the larger the plant is in veg, the more bud you will get at the end of the day.. it's just science, lol. the bigger the plants, the bigger the root system, the more bud you will get..
oh yah, also, it's another rule of thumb to use one gallon of container for each month of plant growth, so if you veg for 2 months and flower for 3, you'd want a 5 gallon container.. of course you could go bigger or smaller, but going smaller will give a smaller yield generally..
hope that helps out some m8, and if you have anymore questions, don't be afraid to ask..