How long will it take to get rid of my speech impediment?

Well, i don't know what my speech impediment is called, but it means i can't pronounce words properly, and my voice sounds a little weird, even when i pronounce it properly. I don't articulate well and i have a bit of a nasal voice. I'm not tongue tied, but i sometimes fall over words, which isn't stuttering, its just it goes weird, but that's since my therapy when i've been trying to fix it. I've got exercises that i'm meant to do every day, that include practicing the words, and practicing articulation as well. If i do these everyday, and have a speech lesson once a week (though she usually is about drama, as she doesn't have time to do it privately for me) how long do you think it will take for me to overcome it? And do you have any tips to help me?