Worrab, precisely this task on my PPL exam didn't take more than 20-30 seconds to do (in my head). Of course, I had four answers to choose from, so in actual operations it would take more to arrive at a (yet unknown) value - but not much more for the kind of precision you may need for a VFR flight, unless you are reeeally pushing some limits for your aircraft.
On second thoughts, for those who really need a flight computer or calculator, I'd wholeheartedly recommend the old Soviet aviator's sliderule, a.k.a. NL-10. It has lots more functions packed, and is an impressive conversation piece :-)
Last edited by Ultranomad; 17th Jul 2009 at 23:16.