How much they should eat is not something you can put a number or amount on. Same with how often. Parakeets are small birds, need a lot of energy and will eat off and on through out the entire day and into the night. Your parakeet needs a lot more in it's diet than just seed. Seed is by no means, a complete diet. If you don't make other food items available, your bird will suffer from malnutrition and possibly liver disease. I'm going to give you some links here that will help you with the other foods you must make available for your bird. You cannot make a total diet change, all at once with a bird. It must be done gradually so I suggest you start right away introducing some of these new food items. Don't be surprised if he ignores them for a while. That's normal as many birds are very suspicious of new things, even if it's something to eat. But be patient and continue to offer a variety of foods. After he starts to eat some of the new things, you can begin to cut back on the amount of seeds that are available each day. I can also recommend LaFeber brand parakeet pellets. Add a few to his seed dish. As he learns to eat some of the pellets along with all the other stuff, you can make your final adjustments. It's my opinion that seeds should not be over about 20% of a bird's total dailey diet. The dry foods should be changed once a day. Be sure you are emptying the dish from the day before, washing it well and refilling. The bowl may appear to still be full but if you check it closely you will see there are only seed hulls left. Parakeets remove the hulls from their seeds before eathing and most of the hulls will drop back in the dish. If someone were to wait until they saw an empty dish, the bird will be starving. The wet foods should only be left in the cage a couple hours. They can begin to grow bacteria that is dangerous for birds. Also make sure you have a cuttle bone in the cage at all times.
Click here: Healthy Food for your Parakeet