There is a UK visa processing time table at https://visa-processingtimes.homeoffice.gov.uk/y/united-states/all

"Processing times vary depending on a range of factors."

As of Feb 2015:

Visit Visas

Most UK visit visas are processed within 10 days, but some can take up to 60-90 days.

Most Business visit visas are processed within 10 days but can take up to 30 days.

Most Family Visit Visas are processed within 10 days but can take up to 30 days.

Most transit visas are processed within 10 days.

PBS Visas

Most PBS Tier 1 visas are processed within 30 days but can take up to 90 days.

Most PBS Tier 2 visas are processed within 10 days but can take up to 15 days.

Most PBS Tier 3 visas are processed within 10 days but can take up to 30 days.

Most PBS Tier 5 visas are processed within 10 days but can take up to 30 days.

Settlement Visas

Most Settlement visas are processed within 30 days but can take up to 60 days.

We know how difficult it is to wait for your decision, especially since you don't know when you'll get it!