Long enough to be tax resident in the UK, ie 183 days. However it can be less depending on how you score on the statutory residence test. It all becomes a bit of a complex muddle, as you can be dual tax resident, so will also need ro declare in France. But if this is potentially a substantial sum then do please take professional advice. Seemingly small differences in your personal circumstances can make a big difference. Don’t rely on advice from strangers on a forum unless they say they are a qualified tax adviser with cross-border experience and passporting rights!
And tax is individual in the UK, not by household. So if the house is in joint names then you each pay your share of CGT. Or would each have to qualify for exemption.