Correspondence courses can also be much cheaper than the CELTA, Trinity TESOL or other college-based courses.

Course lengths vary but the minimum seems to be 6 weeks, up to a whole year or more, depending on how intensively the applicant wishes to study. Many colleges and universities offer these courses via the internet and within regular academic term times.

As with so many things in life, the length of the course is not a sign of quality…

Price is not a water-tight guide to quality. A brief glance at the Internet will reveal courses in the range USD 295 - 8,600.

The best recommendation is probably word-of-mouth. The Internet is the place to (thoroughly!) research these courses for the latest information. You’ll be surprised how many Universities and Colleges you already know offer these courses. Their general reputation is as good a quality guide as any.

He are some other considerations:

If you're not able to pursue a full-time course, a correspondence course might be your best bet. The qualification you receive may not be recognised by all language schools, but it does improve your chances of getting employment, since it demonstrates your interest in teaching English as a foreign language.

If your reason for taking a correspondence course over a college course is solid, any decent potential employer should view your endeavours and qualifications positively.