(non pilot speaking) When asking this question, you may wish to specify the altitude of the a/c, as well as the loading of the machine and the thrust rate that is to be used.
At LHR, a 744 is motoring for (typically) around 45/50 seconds before rotation whereas, at JNB with an elevation of some 5,000' the wheels are working for (sometimes) another 20/25 seconds. In this matter, everything depends upon other factors and there are many of them to be considered.