It will take as long as you allow. It really depends on how determined you are and what you do!
Walking before breakfast is great, just make sure you're pushing yourself harder each day. Eating late in the day is really bad though.
A big breakfast is fine so long as it's healthy, ie. cereal with low fat milk, 2 slices of wholegrain toast, banana, low fat yogurt and an apple is perfect.
Here's a few tips to help speed things up:
Have a no sugar caffeine drink (ideally tea or coffee) before your walk to kick start your metabolism.
Tense your tummy muscles whenever you can and make a special effort to 'hold in' your tummy while walking.
Try to do sit ups or stomach crunches every morning and evening, do one more every day.
Do NOT eat after 8pm.
Drink plenty of water.
Take it easy on carbohydrates and try to go wholegrain where possible.
Later meals should be lighter. Also, check out these recipes when you can
http://tiny.cc/qwhmjy
Good luck!!!