Between 4 and 8 weeks, and usually somewhere around 6 weeks. But it depended on quite a number of factors:
- Wind: sometimes the wind was at your back, and sometimes it wasn't. In addition to the randomness of wind, some patterns were clear. It was considerably quicker to leave North America for Britain than to sail directly from Britain to North America. As a result, a number of commercial vessels made a regular circuit between Britain to the Caribbean to North America and back to Britain.
- Speed of the ship: some smaller ships were much quicker than bulkier cargo ships. The colonists actually used this to their advantage after the battle of Lexington and Concord. When the battle was over, they worried that British public opinion might turn against them as a result of the battle. So they printed a sympathetic report of it, loaded copies of it into a schooner called the Quero, and sent it to Britain. It arrived about two weeks earlier than the British officers' account, aboard the Sukey.
- Volume of commerce: this was a minor factor, but it wasn't necessarily the case that a commercial ship left every port every day for every destination. If you wrote a letter in Charleston for London, you had to wait until a ship was leaving Charleston for London (or somewhere nearby) to send it. During the American Revolutionary War, the number of ships moving between British and American ports declined as well, resulting in some delays.
- Season: In winter, in some places the speed of communication could slow to a crawl. Ship captains didn't want to sail into northern ports in extremely cold weather in case ice had built up. This was especially a problem in Canada, but it also deterred ship traffic in Philadelphia, Boston, and New York.
Because of the slowness of communication, colonial governors and military officers had a significant amount of latitude to respond to changing circumstances. It would have been a disaster if they needed to wait three months to hear back from London. This also made it more difficult for colonists to keep track of London's internal politics and policy-making.
The slowness of Atlantic communication persisted for quite awhile, changing slowly until transatlantic telegraph cables were laid in the mid-19th century.