I get asked this very often. It's a difficult question to answer because ultimately I am going to decide by how I feel and look, but I have set goals and a final target.
I used the Slimming World book to set myself interm targets, all of which I've hit. My final target is on the horizon now.
I have set myself one more mini target in between my final target and where I am now. There is a very important reason behind the mini target that I shall explain more at a later date.
Before I started Slimming World I set myself up a chart of how much I wanted to lose. I drew it all out neatly and popped it in a frame. Each time I lose a lb I stick on a shiney smiley sticker. It is in every sense a child like rewards system, but it helps me so much to visualise where I am, how far I've come, and how much further I have to go.
I've marked my chart with gold stars to represent my next 2 goals. As you can see I'm not far off at all.