TimeUsed[ ] gives the total number of seconds of CPU time used so far in the current Mathematica session.
TimeUsed records only CPU time actually used by the Mathematica kernel. It does not include time used by external processes called by the kernel. It also does not include time during pauses produced by Pause.
TimeUsed is accurate only down to a granularity of at least $TimeUnit seconds.
See The Mathematica Book: Section 2.13.3.
See also: Timing, SessionTime.