There are no actual hard fast rules about this, just guidelines and you can read the UK guidelines
here which is a link to the UK MATS Part 1 (CAP493). Open the PDF document and Section 5, Chapter 1 - Aircraft Emergencies Part 13 is all about Fuel Jettisoning.
The basics are to attempt to fuel jettison above 10000ft, however if not practicable to try and at least be 7000ft during winter and 4000ft during summer.