I Want to Lose Weight

I have just had a heart to heart with my gf and decided that I will lose weight... I gained 90lbs over the last 4 years and had been happy doing so but just lately my feelings towards it have changed a little. I am confident in myself and my body but I can't pretend it doesn't impact on my life. My knees hurt, and we had to pass on a property we looked at getting together as the shower cubicle was too small for me. I feel invisible when I go out, like people just look past me. I want to set a good example for my niece and be able to run around and play with her. Sometimes I feel like I am wearing a fat suit I can just unzip and step out of but it's not, I have to walk around carrying it all the time.

I'm at 220lbs right now and know that the next few months are when it's easiest to gain more so I'm starting now, we're going to give $20 to charity for every 10lbs I lose up to a total of $100. So that's a goal of 50lbs to lose. What is a sensible time to do it in? A year and a half?