Royal Mail's target for air mail going to the USA is for delivery 5 days after posting. Since mail coming in the opposite direction will be using the same systems, it seems reasonable to assume that it will take around the same time.
If youre buying an item from the US it may be delayed by HM Customs & Excise. I recently bought a watch thinking I'd make the most of the exchange rate, but what I didn't bank on was the VAT chargable. I ended having to pay the VAT and parcelforce charges before they would deliver........ In answer to your question it took 7 days regardless.