When I see this comment, I assume it goes with a withdrawal plan that says in "some" circumstances I'd get 100% of my withdrawals from bonds.
If I get into that situation, clearly my "number of years in bonds" will drop. But, that must be okay, or why would I make this type of plan?
So I think this approach requires some explicit plan for when to actually withdraw those bonds, when to stop withdrawing them, and when to refill the bond bucket.
The answer to "how much is enough" comes from writing out that plan then running it through possible market scenarios until I've convinced myself that it meets my goals (whatever they may be) in at least X% of the plausible scenarios.
So I'd say you should start by writing out your plan for using and refilling your bond bucket.