Ideally they'd want Biden for 2 years, since a given person can be POTUS for a maximum of 10. You can be elected twice for 4 years each and serve up to 2 additional years if as VP you replace the President.
That said in theory I rather approve of this proposal, though it's obviously being used for partisan reasons here I think it's a sound idea to have the President get regular physical and mental checks and remove a President who shows signs of dementia or mental illness. Obviously this would need a lot of checks and balances to keep it from becoming another tool to browbeat the opposing party with but again, in theory I think it's a sound idea.