How long do you think it will take now?

okay so my little boy will be 3 years old on March the 29th. and he is just now showing a really big interest in potty training. he loves the potty dance comercial that comes on t.v. so the other day he and i went out and i let him pick out his new big boy underwear all by himself. through the day i have been putting them on him, and i tell him specifically don't let Elmo get wet or dirty and if he needs to go pee or poop to come tell me and i will take him to the potty. he has been doing this just about every time over the entire weekend, but we just haven't been making it in time and he's messed up quite a few pairs of undies. i want to keep at it like this as i feel that he is very close to getting it. he even gets really upset when he doesn't make it in time, he says oh no and starts to tear up. what else can i do or should i be doing to help him get to it in time? he has gone through a lot of underwear and it's tough because our apartment has no washer and dryer so i do laundry at the laundromat once a week. he does have some pull ups, but he really doesn't want to wear them he wants to wear the underwear, so we have only been using those at night.. so my questions are, once your child really started taking a big interest in it how long did it take him or her to really get it? what other things can i do to help him along? i have been trying to do the sticker reward thing, but until he really makes it to the potty on time i can't give him one, although he does know they are there and really wants one. please any advice or tips would be appreciated.. really want to nail this for the most part before his birthday... thanks! =)