I'm Susovan, a Ph.D. in mathematics who is also a certified instructor of ESL. I spent several years in the United States and I've a native/bilingual proficiency in English. I've been tutoring for over nine years and have received excellent reviews! My courses focus more on conversational English and accent reduction, but I can work on your grammar as well. I also give homework on a regular basis to make sure you understood what went on during the tutorial sessions. My sessions are very much tailor-made and can always be adjusted to your individual needs. I tutor in groups as well, for which there can be a price reduction.
I think it depends on several factors.
One year, if one commits.
If the selected tutor is not available, feel free to contact more tutors: