I`m at 91,000 miles now on a 1982 metro 998 engine that is sat in my daily driver.
I don`t use additive ( it costs money ) and it`s still going strong for the last 56000 miles i`ve clocked up with it with equal compression on all cylinders so I would say drive on until you want to modify it or it burns a valve out. if you are pottering around and not clocking up many miles each year you might never need it.
If it`s in bits already that`s a different story as any work on the valve seats will remove any lead memory and you will burn the exhaust valves/seats out in a short time.
I have a factory unleaded head saved back for when I need to change it, that and every one I have had in the past has had both exhaust and inlet seats fitted which seems a bit OTT.