i have always been told landlord has to give two months the tenant one. And on the three times i have been evicted i was given two months notice.
There was a thread on here about the notice being tied to the tenancy period, so it might not be two months, but two months from the beginning of the next period, but you would need to check this.
edit
http://www.tenancyagreementservice.co.uk/section-21-notice-to-quit.htm
quote.
Section 21(4)(a) of the Housing Act 1988 applies to assured shorthold tenancies that have become periodic and states:
“Without prejudice to any such right as is referred to in subsection (1) above, a court shall make an order for possession of a dwelling house let on an assured shorthold tenancy which is a periodic tenancy if the court is satisfied-
(a) that the landlord or, in the case of joint landlords, at least one of them has given to the tenant a notice stating that, after a date specified in the notice, being the last day of a period of the tenancy and not earlier than two months after the date the notice was given, possession of the dwelling-house is required by virtue of this section; and