I’m consulting some other universities’ policies as stated on their websites.
The one at Cardiff is interesting:
2.16 Honorary Professor
The title is awarded to an individual who is making a substantial commitment of a non-transient kind to the teaching and research activities of the University at a level deemed to be worthy of this title when assessed against the criteria for promotions to personal chairs and following the consideration of two external assessors.
Persons nominated could be senior academics at other Universities (that is to say those already in possession of a Chair), Professors who have left the University to undertake work of an essentially academic nature outside of Higher Education, or recent retirees who are not eligible for conferment of the title of Professor Emeritus by virtue of their continued contribution to the University’s activities.
Category B [which is the relevant category]
3.3 The normal period of tenure for all titles in Category B will be five years, though this may be less for those with the “Visiting” title.
All awards in both categories are subject to regular review to ensure that the award continues to be merited. The University reserves the right to cancel an award where there is a sufficient justification.
Heads of Schools should write to the HR Division with a justification for the renewal of the award, which should be based on a continuing contribution to the University. The HR Division may ask for further justification where this is deemed to be appropriate.
4. RESPONSIBILITIES OF INDIVIDUALS HOLDING HONORARY TITLES OF CARDIFF UNIVERSITY
All involvement with the University should be agreed with the Head of the relevant School.
Honorary staff will be required to abide by University policies in so far as they are applicable; and in particular those relating to the Financial Regulations and Human Resources and Health and Safety polices.
[several paragraphs about safety and intellectual property]
At all times Honorary staff will be expected to maintain the good reputation of the University.
It’s pretty clear that at Cardiff, at least, an honorary professorship is not at all like, say, the Booker prize – it’s not something you get as an honor and then get to keep forever because you won it. It’s something Cardiff can renew or not renew, as it decides, and can withdraw if it decides that.