If it's anything like my experience, the answer is 'never'. My 7 yr old is quite content to coast on the trail-a-bike. He always leaves his own bike in the lowest gear and maintains a zippy cadence when he's riding himself (albeit topping around 7mph), but on the t-a-b he keeps it on the top gear and turns the crank like twice per minute. He says he gets better exercise that way and still gets places faster.

Our t-a-b is heavier than our trailer, which makes some nice symmetry w/ the 7 yr old weighing more than the 4 yr old also. The answer is definitely to make the 7 yr old ride his own bike so I can pull the trailer and my wife has no load.