Determine MOSFET switching speed
From the point of view of the driving circuit, the gate looks like a capacitor to the source.  It actually has some capacitance to the drain too, but that has been taken into account in the total gate charge figure.  You know the voltage the gate must change and the charge that must be transferred to accomplish that.  From there it's simple to compute the equivalent capacitance: Farads = Couloumbs / Volt.  Once you have the capacitance, The R*C time constant gives you some idea how fast the gate will slew given a step input on the other side of the gate resistor.  To reach 90% of the final gate voltage, for example, takes 2.3 time constants.

When the FET actually "switches" is more tricky.  The FET won't suddenly go from full off to full on at a particular gate voltage, but there is a gate voltage at which a small incremental change will make the most difference in the FET output characteristic.  You have to decide how full on and how full off "switching" really means, and then decide what gate voltage range that represents.  Then you can use the equivalent R-C model to decide how quickly a step input will cause it to slew accross this region.  For example, if you decide most of the switching happens between 20% and 80% of the gate voltage, then that would be 1.4 time constants.