5 Year Tourist visa in UK on an Australian passport

There is no stated limit to the amount of time that you have to spend out of the country between visits.

However on the return visit, the immigration official will need to determine if you are a bona fide visitor, or if you are attempting to reside in the UK.

The specific section of the UK Visitor Rules that covers this is section 4.2(b) which states :

Genuine intention to visit
V 4.2 The applicant must satisfy the decision maker that they are a genuine visitor. This means that the applicant:
(b) will not live in the UK for extended periods through frequent or successive visits, or make the UK their main home; and

The UK also make details of how their immigration staff make such decisions available, which states:

Frequent or successive visits:
How to assess if an applicant is making the UK their main home or place of work
See paragraph V 4.2(b) of the visitor rules.
You 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 if they have registered with a general practitioner (GP), or 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.