Only about 300 years, from your own link you kindly provided:
I think you need to read that carefully again. Squirrels have been in North America for millions of years before Europeans arrived. The part you quoted was where Europeans took a specific species of squirrel found in North America, the eastern gray squirrel (Sciurus carolinensis), back with them to England.
The rest of that quoted piece talks about that specific species of North American squirrel’s spread around other parts of North American.