70 to 80 Years
Some mortgage lenders will still lend, but many require a minimum of 85 years at the end of the mortgage term. Buyers are aware the lease will dip below 80 years soon, once it does, they lose the right to extend on the most favourable statutory terms. This creates urgency but also hesitation.
At this length, selling on the open market is still realistic, though you may face pressure to contribute to a lease extension. We buy as-is with no extension required.