herhonesty has it absolutely right. Bear in mind that a lot of new builds go with 99 year leases, so a lease with between 90-100 years left isn't unusual.
Having said that, the further away you are from 80 years, the cheaper it usually is to extend. If you go to the leasehold advisory service website they have a good calculator which estimates costs, and a guide to lease extension. It's www.lease-advice.org.
i'd also add that the calculator is fairly misleading - in my case, I'm dealing with extensions which are 10k out. but the key is, the sooner you do it, the cheaper it is regardless of whether calculator is wrong or right.