Depending on the price of BTC in 20 years and depending on your location in the world, If BTC is worth $1000 Then I think a decent amount for a first-world resident would be 1000 BTC, it won't be enough to buy a yacht but would allow reasonable living conditions "if USD hasn't deflated in value that much"
I don't think bitcoins would last long and hence I won't be saving over 10 bTC as of now. If BTC does last for the next 20 years, $1000 would be too less per BTC as by then the prices would be higher and $1000 by then would be worth $100 today. It should be a 5 digit figure at least.