From the Royal Mail website (delivery aims to the USA):
Airmail: 5 to 7 days
International Signed For: 5 to 7 days
Airsure: 4 to 6 days
Surface mail: Up to 6 weeks
Those figures assume that the correct customs documentation (a CN22 form) has been affixed to the parcel. Otherwise it's likely to be impounded by US Customs and never arrive at all.