That's rather a long time!! At Heathrow, provided there are no vortex or separation problems, they aim to get the next one rolling as soon as there is daylight between the wheels and tarmac of the one ahead. In reality it's very hard to achieve much better than a one-minute departure rate, but with a bit of work and cooperation from the crews it can be done... I recall one controller getting 15 away in 12 minutes once...