I am 5ƈ" and weight 190 lbs. I am beginning an exercise plan that involves biking for 30 minutes 1 to 2 times per day. I will be eating a low calorie diet (specifically, 2 slimfasts per day and a sensible dinner). Because I never exercised before, my boyfriend says I could be down to 120 lbs. within three months. I think he's crazy and it'll take at least six months. Do you agree? What do you think? Am I on the right track? Any suggestions? Thanks for the help!

70 pounds in 3 months? That's a signifigant weight loss in that short of time.
Regardless of the type of diet you choose - your body will end up loosing between 2 to 3 pounds a week after 2 or 3 weeks. You might have a 5 to 7 lb weight loss the first couple of weeks if you get lucky.
So - 4 weeks x 2.5 (I'm averaging) is 10 lbs a month weight loss. X 3 months is 30 lbs.
I'd say he's being 'slightly' unreasonable - but I swear, sometimes guys are just like that. My husband picked out a pair of jeans - 3 sizes smaller than I was at the time - cause he just 'knew' they would fit. Glad I love him - kept me from smacking the crap out of him. My husband however - if he's working out/eating right - can drop 20 lbs in a month. Makes me want to shoot him. -)
I would NOT set yourself up for a 'I'm going to be 120 lbs by <insert date>. Otherwise if you aren't loosing as quickly as you 'need' to - your going to get discouraged. And you need to ENCOURAGE yourself.
I would suggest that you look into an alternative form of exercising - biking is great - I absolutely LOVE to bike - but 2 times a day - every day is going to bore you to tears after a while.
I would suggest that you look at possibly alternating walking (at a fast pace, until you get to the point you can possibly run) and biking. Or biking and a pilates or yoga workout. Something that is going to break up your 'routine' so that it's enjoyable. You need to find an exercise that - long term - will work for you.
I'm not a big fan of the slimfast diet - because I think it's more important that you learn to make healthier choices foodwise - as opposed to depending on a can of something. Not only that - but it's processed so quickly - your going to be hungry.
Depending on your normal diet - you might do better on a modified low carb (South Beach) or even Atkins. I tend to trend towards whole food concepts of healthier eating, rather than drastic diets like Slimfast.
The 2 best suggestions I can give you are these:
Measure yourself. Measure your neck, bust, waist, hips, and thigh. Remeasure yourself once a month. Sometimes the scale doesn't move much - but you will lose inches - ESPECIALLY if your exercising.
Buy yourself clothes that fit the size that you are. If you've lost 20 lbs - buy new jeans/shirts/shorts - otherwise you feel frumpy as you lose weight, instead of showing off and glorifying in the fact that your healthier than you were.
Good luck!

70 pounds in three months is highly unlikely, but I bet if you eat right and exercise you could lose 40-50 pounds. Don't start your calories intake too low like 1200. That will only work for a few weeks, and then your body will only start burning 1,200 calories. Eat 2,000-2,500 calories to begin with, and you will still lose weight. Personally, after two weeks I reduce the intake by 100 calories that's what works best for me. Good luck and God Bless!