There is no such thing as "normal execution time" for a testbench.
It depends of course on your simulation system (P3 800 MHz has other
Perfomance than a P4 3GHz).
Further on the complexity of your design, next point is the average
switching frequency for your design, as a simulator needs to calculate
every signal switching.
Another point is the testbench itself. You could programm fast
testbenches or slow testbenches. A testbench using only sequential
signalassignments an wait statements will be quite faster than a
testbench, that calculates the stimuli and waittimes or reads them
from external stimuli files.
