Seeing the last 50 years fly by, I think at least another 10x that would be my wish; so 500 years. I suspect it depends on your outlook. Some people go out of their way to be miserable while others have figured that contentment is more of a choice than affliction.

If greatly extended life...along with 'youthfulness' were possible and practical...it would bring a whole new meaning to the phrase....'grab a granny' ;-)

Mirror link: https://www.youtube.com/watch?v=GoJsr4IwCm4

The social implications of vastly increased lifespan are immense. Where is all the additional food going to come from for this rapidly expanding world population? Where are all the jobs going to come from? And where is everyone going to live?

What's the youngest you can die of old age?

#5 Benjamin Button was just an infant when he died of old age.

#5 and #6 Good answer to a good question!

#4 don't forget children. people are still going to want to have children. it's immensely rewarding and gives life meaning. Not to mention it is one of the strongest urges in our (and any organisms!) biology. At one point, the state would have to start regulating who gets to have a child due to room and food constraints. Possibly you would have to pay money for each next year of life past some maximum age.
The social implications can not be understated. It would profoundly change and upend all of our social order. Doesn't sound nice to me!
That said, this will be just as inevitable as self-driving trucks taking away jobs from pro drivers, development of general AI and genetic engineering of humans (just to name a few). Technological progress can't be stopped. Maybe just slowed down a bit. Slowing down seems like a good idea to me.

We would need a world cull to get rid of 99% of the population then the remaining 1% could live forever and have an awful lot of space.

If men is made immortal, eventually he will die by committing suicide.