Every season, I am quite sure.They re-lay the pitch every 2 seasons.
I read an article about Darren Baldwin who said it's stripped (taking 3 days) at then end of the season and is playable after 10 weeks. They use, as do others, ultra violet light rigs to grow in adverse growth conditions. We use a Desso GrassMaster hybrid pitch, as do Liverpool, Arse and a few other EPL clubs.