What is your (FI) problem?  How often do need to diagnose your fuel
pressure?  You certainly can leave a guage and hose on the tee as long as
you like, but I think you're going to see only what the regulator is doing.
The pump puts out excess pressure, the regulator sends the rest back to the
tank.  You can remove the vacuum line and see if the pressure jumps up like
it is supposed to...   If the pressure is too low or too high,  I would
start by replacing the regulator.

                                                             Jay