That depends on your goal.
For one process it's obvious that $t$ is the wall-time of the entire program launch.
For two processes, you got $t_1$ and $t_2$ which correspond for the walltime on processors 1 and 2.
Usually, one is interested in the total time it took to solve the problem - aka overall walltime, which will be $t=\max(t_1,t_2)$. However, I would recommend some additional measures to do a proper job:
MPI_BARRIER before and after the computation is done.
Measure the walltime only on the master node, which better be done right before and after the barrier. Then, you will obtain the accurate walltime of the
ParallelMatrixVectorProductRotuine(...); for $N$ processors.