It depends on rather a lot of factors -
how long I am in the garden for
if there are any little, new and easily scared young ones in the herd
if there are any delicate tummies (not too much grass)
if the weather is on the cooler side and there is an oldie in the herd
But if I can then they go the full forty minutes. Time to fill up the tummy, snooze and let it settle, then get a bit more grass on top. When they come in again they are completely flopped for a while!