Since the rate of grass growth is variable, being dependent upon weather conditions, the answer has to be 'whenever you think it's necessary, to keep it the way you want it'.
Given the recent weather it'll be a case of grabbing the moment, I'll be hunting tigers in mine soon! Sorry muchlovex, honestly not making fun of your serious question, just I'm desperate to sort mine out but the wet stuff keeps on falling!
During a drought (not this year then!) some of my neighbours will go out and cut their lawns once a week. We don't, and when we do we have it on a longer cut setting so it doesn't dry out and go brown.