Bonds are issued for specified times, anywhere from 1 year to 20 or 30 years, at which time the bond principal is paid back. Short-maturity Treasury bonds are called T-Bills, with 1-year maturities. They have historically been among the safest of ways to invest money, and this is reflected in the small interest payment one receives. Longer-maturity bonds are known as T-Notes and T-Bonds, and are used to hedge interest volatility and reduce risk over multi-year cycles (among other things).
In practice, bonds are rolled forward. As one set matures, the proceeds are often used to purchase new bonds. Bonds short of maturity may be sold to other parties at market prices at any time. Repayment of the bonds cannot be demanded of the issuer until the maturity date.
There's a decent overview of how this works here:
http://en.wikipedia.org/wiki/Treasury_securityApart from the impossibility of "demanding" repayment, China does have the option of not buying so much US debt in the future. That would force the US government to offer higher interest to attract replacement buyers, and would harm the US budget (there's a good overview of the national debt here:
http://en.wikipedia.org/wiki/United_States_public_debt ). However, consider that China emphatically does not want "to turn America into the SS Titanic," because that would deep-six its own economy, and China would come apart at the political seams. Most political observers believe that if China can't maintain, at bare minimum, about 5% growth in its economy (it's been averaging over 10% for fifteen years) then the forces of rural and peasant unrest will reach the boiling point.
China will be lucky to see 6% in 2009. If the US stops buying their stuff, that 6% or anything near it goes poof.