So already something like half of our lives are covered by some kind of UBI. 20 years to mature, 20 years of retirement, sandwiching a 30-40 year career.
Don't forget, UBI would replace everything else. Food stamps, unemployment benefits, social security, etc etc. And literally SQUARE MILES of office building staffing bureaucrats who do means testing, fraud prevention, etc etc for government benefits would also not be required.
If you put together all of the money we spend for all of that, I wouldn't be surprised if 90%, or even 100% of the cost of a UBI could be covered.