I worked the cut off point for ourselves driving down v flying and car hiring, to be 3 weeks, if we would be in Spain any longer than 3 weeks (excluding school holidays) it was cheaper to drive down and use my own car than to fly and hire.
With regards to leaving the central heating on un the UK when you are gone. My Mother fell foul of this when she went to Benidorm for 6 weeks in Jan Feb, many years ago, the boiler packed up even though I visited the house every week, It still did not stop £20k of water damage that the insurance would not pay up on. Personally I`d turn the water off at the mains and drain all the pipework down including central heating, it`s the only way you can be sure. If you have a conventional boiler set up you need to leave the mains turned on to top up the header tank for the central heating, not sure about these new fangled combi boilers.
I`d also get someone I trust to stay a couple of nights a month to satisfy the insurance company.
On a side note which may be of use to someone. Last year we drove down to our place and stayed for 6 weeks on the way back the car overheated in the Pyrenees when I called the breakdown cover I had, the first question they asked was on what date did I leave the UK the policy was an annual policy but in small print only 30 days at a time, so I lied, eventually got recoved to the nearest Skoda garage in Huesca at 7pm the manager filled the car with antifreeze ran it for half an hour said the fan was on it`s way out but if we drove the mountains with the heater on full it should not overheat again, he would`nt take any money, we drove through the night and made the ferry.