By definition online courses are flexible time wise. Online courses allow you to study while holding onto your daytime job or taking care of your family, or fulfilling prior study commitments.
Completion time for our 100 hrs TESL/TEFL Certificate, for example, depends mainly on you. We work at your pace and you are not bound by our timetable. The structure of our program allows our students to start the course and graduate any time they want. Average completion time is around two months, but we have had students complete the course in 25 days and others who took just over two years. So it’s very much up to you!
The standard minimum requirements are a BA plus a TESL/TEFL Cert. and/or experience. Of course, as with any job, the more you have to offer in terms of qualifications and experience, the better your chances of landing a well paid job with opportunities for advancement.
If you are looking at a position in one of the International schools then you should be qualified to degree level with a recognized teaching qualification at degree or post graduate level. Usually these schools give preference to people who have qualified in one of the following countries: US, UK, Ireland, Australia, NZ, Canada or South Africa.
If your aim is to work only with private students then your ability to deliver effective lesson in a friendly yet professional manner will count more than your qualifications.
Since you area already in Dubai I’d recommend you do a bit of ground work and research the market. Being yourself on the spot and with your sister already teaching there you should have no problem gathering the necessary information. Start by visiting a few private language schools in the area and see what there requirements are, keep an eye out for classified ads in the local press, etc