In reply to TobyA:
> When it's knackered.
Indeed
> When you start worrying about it - might be the best answer.
More often.
To be honest, you should get a new harness when:
1) belay loop is visibly frayed/degraded/knackered.
2) the waist tightening strap is visibly frayed/degraded/knackered.
3) legloops are visibly frayed/degraded/knackered.
4) you feel like it.