The longer you can live in a property, the better but there is no set minimum. It's more about the "quality" of occupation as opposed to the period of time.
If you sold the property and made a gain of £140,000, having never lived in the property, the first £11,000 of the gain would be exempt from CGT. That would leave £129,000 taxable at 18%, 28% or a combination of the two rates depending on the level of income in the tax year of disposal of the property. If you sold the property in the current tax year, one of the following scenarios would apply:
1 If your income in 2014/15 including the taxable gain is £41,865 or less, then all the taxable gain will be taxed at 18%.
2 If your income in 2014/15 excluding the taxable gain is more than £41,865, then all the taxable gain will be taxed at 28%.
3 If your income in 2014/15 excluding the taxable gain is less than £41,865 but more than £41,865 when you include the taxable gain then part of it will be taxed at 18% and part at 28%.