Two minutes per question is the general rule. You can keep track simply go looking at your time every 10 questions. 10 questions, 20 minutes. Or 5 questions, 10 minutes. This is quite simple and requires very little thought which translates to less distractions during the exam.

Pacing is very important to survive the GMAT.

What I did was draw a time reference chart like this:

Math
problem#--------------10---------20--------------30---------35
time remaining----~55min-----~35min------~15min-----~5min


Verbal
problem#------------10------------20-------------30-----------40
time remaining----~57min-----~40min------~20min-----~2min

Of course none of this is set in stone.