Allotments 4 All

My son has just moved house and inherited two lawns which are full of dead grass and moss.  If he used something like a Flymo lawnrake on them now, how long would it take for them to get back to normal?  Would the end of August be unrealistic? 

Judging by the reviews of the Flymo Lawnrake it would break down before the job was finished.....

Las time I dealt with a lawn like that I used a wolf scarifying rake to clear the moss & thatch (surprising how much there was, I filled 3 FIBCs with moss & it was a fairly small lawn), did a bit of spot weeding to remove dandelions, docks etc... then aerated it using a garden fork & spread a 1/2" thick layer of sifted compost over it.
Within a week it began to go green again, 2-3 months later it was very densely growing grass & looked like a healthy lawn.