There is not a hard and fast number of days rule. However on each entry you need to satisfy the officer that you do not intend to live for extended periods in the United Kingdom through frequent or successive visits. This will become more difficult each time you keep adding long stays in your past immigration record.
The old rule of thumb was no more than 6 months in any 12 but the number of visits probably also come into it.
There is no longer a visa category for retirees with own means - this was suspended in 2008 and there is little prospect of this coming back.