Never is the answer. Americans should not have priority over anyone anytime according to free trade economics.
Free migration leads to the highest productivity for the world, which helps create jobs for those poor unemployed Americans, as well as paying for increased wages and social spending.
France tried lowering the work week to create jobs, and it didn't work which should tell you that there is no set amount of work to dole out to Americans. Your idea will contract the economy like happened in France, with no net benefit.
Unemployment is a natural part of the economy, and foreigners should not be scapegoated for it. Economies expand as populations expand. How else could the U.S. population go from 0 to 300 million since 1607 while dealing with unemployment and recessions that whole time.
The best way to understand how misleading your question is is to extrapolate:
1) How many people in California have to lose their jobs before they are given priority over New Yorkers?
2) Then you could say How many people in LA have to lose their jobs before they are given priority over people from San Francisco?
3) Then you could say How many people from East LA have to lose their jobs before they are given priority over people from West LA?
By extrapolating you approach the point of absurdity. If you are to be taken seriously, we would have to stop the 300 million Americans form competing with you for jobs also since you believe we have to stop overcompetition.
I'm Canadian, and there are still a lot of jobs for you here in Alberta, Canada (jobless rate has been 3.5% because of oil boom). Oh yah, but you don't believe in free migration, so I guess you will just have to sit on your unemployed but in the U.S. complaining about foreigners. If you want to stop Canadians like me from competing with you, even though I'm indistinguishable from an American, then I guess we will play tit for tat.
Even if we stopped all foreign competition in the U.S., you would still be competing against foreigners overseas unless you think we should stop all trade to please you, which would be absurd.
You will have more pride in yourself if you get your own job rather than asking your government or family the to guarantee you one.
Now stop whining and give me my best answer.