Legal time to turn down music in surrey?

The law doesn't specify a time. Your enjoyment of your premises has to be balanced with your neighbours' enjoyment of theirs. The law tends to be based on what is reasonable. Generally speaking, the volume shouldn't be sufficient to cause inconvenience after around 9:30pm and it should not be audible to your neighbours after 11:00pm. However, if it's extremely loud even in the daytime, it could be considered a nuisance and your local authority are empowered to confiscate your sound equipment.