Lawn Mowing Height
Your first light mowing should take place after the turf has taken root. You will know this has happened when you can no longer separate the turf from the underlay. If you allow the turf to get too long don't mow it any more than 1/3 of its height at a time.
Mow it 2 or 3 times over a week or 10 days, gradually lowering the mower each time. If you don't obey this rule, you could find you will get a lot of dieback in your lawn, especially in humid weather.
Mowing heights can vary from 15mm to 50mm. However correct height can depend on personal preferences. Mow at the higher level in shady areas and also prior to winter. Generally a longer leaf will encourage the root system. In the early spring when the turf has started to grow again you can mow the turf short a couple of times to get rid of the thatch (remember the "1/3 of it's height" rule).
It is very important it is to keep your mower blades sharp and in top condition, as blunt blades will also cause dieback in your lawn, once again especially in humid weather.