Hi dustpine,
We cannot advise how long, but if the property is your mother's main residience for the entire period of ownership then there would be no tax payable. As you mother is not UK resident while owning this property, that will not be the case.
The UK property would only become her main residence from the date she moves into the property. This would have to be declared in writing to HMRC.
Private Residence Relief is the fraction of time the property is the main residence over the period of ownership, usually calculated in months.
Have a look at the help sheet HS283:
HS283 Private Residence Relief (2024)
Thank you.