How do you mean? If the Fed creates new money and buys bonds with it, the Fed can continue to hold them forever (really just until they mature) if it wants to. Then the Fed is more or less part of the government, so when the bond matures the Fed takes the bond to the treasury which gives it cash, then it gives the cash back to the treasury. It's like the bond evaporates into nothing when it matures while the Fed is holding it.
If the Fed theoretically bought all the bonds then it could obviously no longer increase the money supply by buying bonds, but we're a long way from that, and even then the Fed could still create new money and give it directly to the treasury to spend in lieu of collecting taxes. Or in the utopia where buying all outstanding government debt and funding the entire federal budget out of new money still hasn't caused the desired amount of inflation, to use to enact a negative income tax.
If the Fed theoretically bought all the bonds then it could obviously no longer increase the money supply by buying bonds, but we're a long way from that, and even then the Fed could still create new money and give it directly to the treasury to spend in lieu of collecting taxes. Or in the utopia where buying all outstanding government debt and funding the entire federal budget out of new money still hasn't caused the desired amount of inflation, to use to enact a negative income tax.