Probably about to the amount of working time and thus income it replaces? That could be an hour a day, a day a week, or a week per month depending again on how much UBI we're talking?
"Having a minimum wage job that that keeps you at the the poverty level doesn't really dissuade people from wanting more and trying to get it."
That's because the job isn't keeping someone at the poverty level, they are keeping themselves at the poverty level by choice. Sometimes, some people choose to change that because they realize they are the only thing holding themselves back, not the other way around.
"The entire culture of having a "career path" with an upward trajectory runs counter to the idea that people, given something like UBI, will suddenly stop wanting & being willing to work for more."
Agreed...although many in America are already choosing to reject the notion of a "career path" and have relinquished their desires for "upward trajectory" - and that's before UBI.
Many people will work fewer hours or quit their second jobs. Currently 7 million workers have a second job.[1]
Young people graduating from high school and college would have a greatly diminished incentive to enter the workforce. Why get a job at all when you can shack up with your buds, pool your UBI, and have all the money you need for booze and video games? Many will fall into this trap temporarily, some permanently.
Millions of creatives who want to work on their music or write the next great american novel are out on day one.
How about the folks currently working to supplement Social Security? Currently, 10 million people over 65 are still working[0]. How many of them quit their jobs on day one of UBI?
Millions of working moms are out on day one.
The biggest issue will be the fact that UBI will make retirement instantly possible for many, and much easier for many more. Currently, to retire with an income of 40K, you would need a nest-egg of $1.3M (assuming a conservative withdrawal rate of 3% annually and no social security). But if you're guaranteed a UBI of 12K per year, a married couple receiving 24K per year in guaranteed income is able to retire with a nest-egg of only 533K. Tens of millions of the most productive workers in the economy will be ready for retirement decades ahead of schedule.
None of these things are necessarily bad--I'd love to be able to retire a decade ahead of schedule! But when the workforce is significantly reduced in size while spending is dramatically increased, taxes will necessarily skyrocket, the incentive to work will plummet even further, and the resulting feedback loop will hollow-out the workforce and bring the economy to a screeching halt.
[0] https://unitedincome.capitalone.com/library/older-americans-... [1] https://www.wsj.com/articles/does-everyone-have-two-jobs-153...