I am a person who likes to see thing on paper and who likes a schedule. This is what I did - maybe it will give you a sort of sense of the time needed to lose the amount of weight you'd like to lose in total. It can then help you set the mini goals, too.
I wanted to lose 100 pounds (from 252 to 152) and hoped to do it in 11 months (mid January to Christmas.) I made an Excel spreadsheet and entered the date of every Wednesday (that's just my choice for weigh-ins) down the column. I figured that with such a large amount of weight to lose, I could probably lose 3 pounds a week for the first 10 weeks, so I put that in the formula and started with that. That took me to the week I was visiting my DD in NC - so 30 pounds by late March was my first mini goal.
Then I figured I could lose 1% a week after that until I hit my goal. The 1% would go down each week as my body weight got less, ending with about 1.5 pounds a week at the end. I didn't know where this formula would end, but I thought it was a realistic amount of weight to lose each week if I stayed on plan. When I put in that formula, MUCH to my surprise, it ended with 150 pounds on December 21st! Exactly what I wanted! Well, that seemed perfect if I could stay on track. I also allowed me to see where my projected weight would be on any given Wednesday. That helped me set other mini goals, like 50 pounds by the time I went on a camping trip with friends, 75 pounds by a family wedding, etc., with lots of smaller ones in between.
This was an ambitious schedule, but it worked. I think it worked BETTER because I could see the whole thing on paper. It held ME accountable to ME every Wednesday. Not just hoping the scale would show a loss, but I wanted to see a specific loss and worked hard to meet it.
Not everyone will lose at the same rate. Not everyone has the same set of circumstances that allows or hinders one to stay on plan and focused. Not everyone has the same ability to get the mental part of the journey in place. BUT - everyone has the ability to succeed at this at their own pace and on their own terms. All that said, this would be my best advice for someone just taking the first steps on the healthy life style journey:
Exercise along with changing your eating.
Make a plan and try not to deviate from it too much. Healthy eating has to be the way we eat forever, so we might as well get used to it. You can have occasional off-plan days, but be careful of them and get back on plan right away. This is what thin people do all the time. We will be the thin people soon. Think and eat like a thin person.
Set a scheduled goal you know you can meet and then do all you can to actually meet it. Losing 100 pounds is so huge - break it down into smaller goals that don't seem too bad. ANYONE can lose 10 or 15 pounds. Just do it over and over until you hit your goal.
If you are a visual person, like I am, make a chart and STICK TO IT. It is a new mini goal every week in the simplest form. Writing it down made it very real for me. It also made me see that it was probably possible to do it in the time frame I wanted.
Make your biggest goal be to simply stay on plan. The weight loss will come.
Come here for support EVERY DAY. Join a challenge or 2 and see how it goes. The Thanksgiving challenge is pretty active. It's still 10 weeks until Thanksgiving - 20 - 30 pounds would be reasonable for most people just starting because motivation is high and there is a lot to lose right off the bat. When you meet or exceed a mini goal, your motivation and sense of purpose just go sky high and pull you along to the next one.
Realize that there is no lasting joy / comfort / satisfaction / etc. in going off plan. Staying on plan will bring you more of these things than any off plan choice will ever bring. Get that in your head, and things will go a little easier.
I hope some of this will help you. I apologize for being so long winded. Welcome to the forum. You're going to find lots of support and help in here.
Life style change started on Jan 13, 2011. I was going to lose 100 pounds by Christmas.
I lost 93 pounds by Oct 1, 2011 and am holding there for now. We'll see what happens.
New goal: To maintain at about 160
Final Goal: To decide if I need to lose more
Just Keep On Keepin' On