If you want to take an ultra-conservative approach, then 7 years from retirement, transfer 7 years worth of expected draws against your portfolio into bonds. If the market crashes the day before the "7 years from retirement" point, then wait 7 years to make that move into bonds and do it just when you retire.

If things are in such horrible shape that the market fails to recover over any given 7-year period of time, expect shotgun shells and canned food to be the new currency anyway.