Seriously, ???
How many miles you drive is entirely up to what you do. If you're only going to drive 5 miles to work probably nto that much. If however you have a liking for the highlands of scotlands and intend to visit every free weekend, you're going to be doing a hell of a lot more.
How long is a piece of string question ?
For reference, average milage of motorists in britain is about 14,000 miles a year iirc
As for insurethebox, check around. Get lots of quotes and see exactly how much cheaper it works out at. Dont just use comparison sites, go to the sites directly and see if the quotes are any cheaper.
Try quotes from people that aren't on comparison sites like Direct Line.
A large portion of the "savings" offered by insurethebox are meaingless. The key seems to revolve around "only paying for the miles you use" but most insuers rarely charge any significant extra premium for say 15,000 miles a year over 6,000 miles a year.
And every other insurer rewards you for being a careful driver, its called your No Claims Bonus !
Not convinced at all.