TimeConstrained[expr, t] evaluates expr, stopping after t seconds.
TimeConstrained[expr, t, failexpr] returns failexpr if the time constraint is not met.
TimeConstrained generates an interrupt to abort the evaluation of expr if the evaluation is not completed within the specified time.
TimeConstrained evaluates failexpr only if the evaluation is aborted.
TimeConstrained returns $Aborted if the evaluation is aborted and no failexpr is specified.
TimeConstrained is accurate only down to a granularity of at least $TimeUnit seconds.
Aborts generated by TimeConstrained are treated just like those generated by Abort, and can thus be overruled by AbortProtect.
See The Mathematica Book: Section 2.13.3.
See also: MemoryConstrained, Timing, $IterationLimit, $RecursionLimit, Pause, Abort, TimeConstraint.