<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by Dave Noble:
It is aproximately 1hour by coach between the 2 airports. The coaches run approximately every 15 minutes. The schedules and booking can be found at www.gobycoach.com.
Dave</font>
However, the majority of the trip is on the M25 beltway around London. If your trip is anywhere near the morning or afternoon rush hours, plan on more time. Perhaps a LOT more time.
There are alternative ways of transferring, involving trains into and out of London, that take about an hour and half but never take much longer (barring strikes, accidents, etc.) at any time of day.