Your thesis should be submitted (both paper copies for examiners and the electronic copy for Turnitin check) after you complete the minimum period of study and before the end of the maximum period of study for your qualification.
For full details on minimum and maximum periods of study, please see University Regulation 6.2.3 (PDF - 129KB). If you are handing in the signed form, you should do so via the Student Enquiry Services counter, located on the ground floor (GC) of the Main Library (R22 on the University map).
Ultimately, the decision to submit must be yours. You should discuss this with your supervisor, but please remember that their advice and/or opinion are advisory only. This advice should not be taken as an indication that the final thesis will necessarily fulfil the requirements of the degree.
Exceptionally you may be able to submit your thesis early.