Down-sizing sale gap and CGT

My mother is looking to downsize and we want to check whether we understand the CGT implications of this downsizing. Her current house was bought in 1980 and has increased considerably in value. This has been her main residence since she bought it. She would like to buy a smaller property, and slowly move, allowing time to sort through her possessions etc. As we understand it, if she sells her first property within 9 months of moving to the second property there is no CGT payable, due to primary residence relief. However, it is likely she will need longer to sort and sell her first property. If this is the case, and it takes 2 years for example, to sell the first property, what is the CGT implication? Will she owe CGT for the period 1980 to the date of sale (i.e. 46 years). Or will she owe only a percentage of this CGT based on the 2 years it was not her main residence? (i.e. 4.35% of that CGT)

Hi
You can work out entitlement (the proportion of your gain you get relief on) to Private Residence Relief when selling your home here: Work out tax relief when you sell your home