When is a good time to leave, and get a new role? It's a big career question and quite frankly does not have a simple answer.
Careers can be made, enhanced, or hit by the timing and choice of change. A key thing is to try and control the 'controllables'. For instance, have clear personal and business objectives clearly laid out, and be aware of how you are tracking against them. A possible 'uncontrollable' is a negative culture that you feel is beyond your ability to influence positively, particularly for your own well-being.
Understand your organisation and its culture, your ability to lead, your ability to influence change and strategy. Talk to your Board or leadership of what would inspire. If what you receive is not inspiring, then it may indeed be time for change.
Change is good if done correctly and for the right reasons. Of course, sometimes it can be forced through events beyond your control (e.g. Ownership merger and/or resulting restructure).
Update your CV at least once a year, challenging its content, but also its style. If it is too hard to update, it might be a sign you have some early signs of career stagnation settling in.
Additionally, stay networked in your local personal, family and business communities including with your favoured recruitment agents.... sometimes change comes knocking on your door