i am a uk citizen-if i visit the usa i can stay for 3 months for a holiday say and then have to leave--how long can a us citizen stay here for in a similar situation--also how long must that person be out of the country before she/he can return for another maximum stay?
Generally, visitors to the UK can stay for 6 months, subject to certain preconditions - eg that they have sufficient money to support themselves, and don't intend to reside permanently. There appears to be no specific rule on how long a visitor needs to stay out of the UK, before returning for another visit; and possibly this would be at the discretion of the Immigation Service. (See http://194.203.40.90/default.asp?pageID=109
for further information.)
Here's a tip: I know that some tourists go over the border to Mexico just to come back in again and have their passports signed for another 3-6 months.