Burning Speed	Expected Burn Time	Burn Time Achieved
1x	60 minutes	60 minutes
2x	30 minutes	30 minutes
4x	15 minutes	15 - 16 minutes
8x	7.5 minutes	8 -10 minutes
12x	5 minutes	7 -9 minutes
16x	4 minutes	5 - 7 minutes
18x	3.3 minutes	5 - 6 minutes

Dual layer media burn times are approximately double the above times and currently the burn speeds are slower.

As you can see the actual burn times against expected burn times increases as the burn speed increases this is because when the drive starts burning it cannot burn at the full speed and has to build up from a slower speed of 4x 6x or 8x achieving the maximum speed only at the end (outer edge) of the burn. The drives have various ways of achieving the maximum speed CAV, CLV, PCAV, & ZCLV these are explained in my article here and the speed transition points can have a great effect on the actual recording times. I have found that the LG drive burning at 16x speed is quicker than the Samsung at 18x speed. & in my opinion 18x burning is a myth until such times 18x speed can be achieved earlier in the burn.
The following two graphs show my point that 16x can be faster than 18x

16x burning with LG took 5 mins 16 secs

There are other factors governing burning time such as disc initialization which can be from a few seconds to a few minutes, burn quality control can increase the burning time whilst the drive selects the optimum burning speed, buffer under-run caused by external processor usage restricting the file transfer speed.
Although I test all my drives at maximum burn speed I prefer to burn at 8x speed which I find tends to result in a better quality overall burn