I can't seem to find information in the FMC tutorial for the PMDG 737 NGX on how to determine total time for flight, or the time remaining before reaching destination. Can someone please direct me to where I can find this information? In long haul flights, I obviously don't sit by my desktop throughout the flight. Instead, I "approximate" the time that I need to return prior to descent instruction by ATC, but sometimes, I miscalculate and overfly the top of descent. Thanks in advance.

Once you reach your cruise altitude, on the PROG page, there will be a time to T/D.  This will tell you what zulu time the FMC is planning to start the descent.  ATC may have other plans for you though.  I use Radar Contact and it usually starts me down 40-60nm prior to my scheduled TOD.  I believe you'd also be able to get this info from the LEGS page as well.  There should be a TOD listed there and it should tell you what time(zulu) that the FMC expects to reach this point.