on average how long would it take me to copy a 300 page book. Its not for school or anything, just want to improve my writing skills?

Answer 1

Why does it matter "how long" it would take? Is the goal to spend a fixed amount of time writing, or to copy this particular book, or just to improve your writing ?

I assume you mean your hand-writing, or as they say in the US "penmanship".

Answer 2

If you want to improve your actual writing - the way your form your letters - you will find excercises on line that would be far more useful than copying out 300 pages.

If you mean writing fiction/non fiction, the time spend copying out your 300 pages would be much better spend reading 300 books.