My Thai niece is planning to study English in the UK for 9 months starting 1 September. I would like to get the visa application sorted asap. Is there a date I need to wait for before we can apply? eg. 3 months or 6 months before entry?

Up to three months before the intended date of travel.

http://www.vfsglobal.co.uk/thailand/

Has she been offered a place on a course in the UK at an accepted place of study?

She must have such an offer before she applies for the visa.

See Student Visitor visa fo details of the requirements etc.

Remember, too, that as she will be applying to enter the UK for more than 6 months she will need a TB certificate.

Thank you for the replies.

We have a school chosen. They will issue the relevant document for a visa once they have received full payment for the 9 month course in advance.

Thanks for the links. I remember now that she can apply for a Student Visitor visa rather than a Tier 4 (General) Student visa.