You cut when it needs it, so when its about 6 inches high. Its usual to put plenty of Growmore or something similar beneath turf before you lay it - if you did not do this, feed with a Lawn feed after the first cut, but according to the instructions on the product you buy - usually you need to wait 4 days after cutting before application, and you need a fine, dry day, and it may need watering in if it does not rain within a given period of time. These come in granular or liquid forms.

Perhaps I am stating the obvious here, but make sure your mower is set at the highest setting so you do not scalp the young grass. Try a small section first to see how it goes. With all this wet weather I would think it is probably ready now for a gentle trim.