看错了，明白了
每次进关，逗留时间应该给6个月吧。
Frequent or successive visits – how to assess if an applicant is making the UK
their main home or place of workYou should check the applicant’s travel history: how long are they spending in the
UK and how frequently are they returning? You must assess if they are, in effect,
making the UK their main home.
You should look at:
• the purpose of the visit and intended length of stay stated
• the number of visits made over the past 12 months, including the length of stay
on each occasion, the time elapsed since the last visit, and if this amounts to
the individual spending more time in the UK than in their home country
• the purpose of return trips to the visitor’s home country and if this is used only
to seek re-entry to the UK
• the links they have with their home country - consider especially any long term
commitments and where the applicant is registered for tax purposes
• evidence the UK is their main place of residence, for example
o if they have registered with a general practitioner (GP)
o send their children to UK schools
• the history of previous applications, for example if the visitor has previously
been refused under the family rules and subsequently wants to enter as a
visitor you must assess if they are using the visitor route to avoid the rules in
place for family migrants joining British or settled persons in the UK
There is no specified maximum period which an individual can spend in the UK in
any period such as ‘6 months in 12 months’. However, if it is clear from an
individual’s travel history that they are making the UK their home you should refuse
their application.
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/581550/Visitors-v5_0.pdf 第12页
这个文件说的很清楚没有在任意的12个月里不能呆超过6个月的说法.但是你要能证明过去一年中你在自己的国家要比在英国呆的时间长.这个过去一年根据我的理解是从你这次入境的时候或者签证的时候算,而不是从任意时间算,比如离境时间.这样其实很多实际造成某个12月期间超过6个月是可行也是合法的.所以楼主父母是肯定可以呆180天的.
谢谢你详细的回复。明白了 
谢谢各位帮忙 
看到这里我笑喷了
是到5月啊 5月到5月一年啊