how to calculate the execution time?

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.