but the magnetic field could weaken in 1.75 to 3.25 billion years, making the surface uninhabitable sooner.
Which is completely meaningless, as in around 1 billion years (give or take 100 million years or so) the sun will increase in brightness and energy output to the point that the planet will be uninhabitable by all but extremophiles living under the surface of the planet. And that would be without any possible reduction in the magnetosphere.
This is no insult to you at all, but I often shake my head at how little most understand how "little time" is left on our planet for living organisms. Or how exceptional our planet is in both placement and how events early on in the history of the planet shaped what is here today.
Such as our sun and planet forming some 4.6 gya, and only around 100 million years later our planet colliding with a similar planet and leaving us the legacy that allows life to exist as we know it even today. Where as the closest planet to our own in such capabilities had their core "die" and the magnetosphere faded away some 3.8 gya.
From start to end, the history of life on the planet will be no more than around 6 billion years. Short of extremophiles trapped under the surface whenever the planet breaks apart.
Because no matter what, ultimately entropy wins in the end. Even the universe dying in around a googol years based on heat death and proton decay.
So remember to set an alarm for around 10,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 years from now to remind yourself of this. Then you can watch the universe die at Milliways. -laugh-