Editors can be very busy and book up quickly. You should book an editor at least one month in advance of when you require the work done. The time taken for the editing process depends on the size of the document and the scale of corrections, however you should expect that an editor would need 3 weeks at a minimum to review your thesis.
For information about engaging a professional editor for your Higher Degree by Research (HDR) thesis, please refer to HDR Thesis Rules and the Preparing for HDR thesis submission webpage (under Having your thesis edited).