The most I've lost is 60 pounds, so I can't speak to the 100# issue.
I can however speak to time based goals. For myself, I do not like them.
Rather than giving myself time based goals, I focused on losing 10 pounds at a time. I call it the decade lifestyle.
A decade at a time. I never cared how long it took me to lose a decade, but I rewarded myself with a pedicure or hair cut or new shirt or new pants, or new kitchen gadget or got the dog a new collar or the grandbaby something, or a new workout outfit or scratch tickets or what ever for each 10 pounds.
All the while, after many years of losing and gaining, the last time, I kept my focus on a decade at a time with maintaining and learning to live a maintenance lifestyle being my ultimate goal.
I spent 10 months losing 30 pounds, but I'm happy to say, I have maintained that loss since September of 2011, I feel good, most days I make good food choices, I can go out to eat, and make better, even if they aren't the best, they are better choices, and I've learned, I love to workout.
If you really want to set a time based goal, it needs to be the rest of your life, not a number of pounds, or time worked out, or carrots eaten or whatever. It needs to be real and reality based.