From the Seathwaite / Honister road junction, it takes around four hours to do the whole loop (depending on stops and fitness). If you’re riding from Keswick, and if you stick to the tarmac heading out and back, maybe add another hour / hour and a half.
If you’re parking in Keswick, I’d maybe save adding any sections of the Borrowdale Bash for the return journey – see how you’re legs are holding up 🙂
Parking can be tricky in town – this time of year, and if you don’t mind paying, your best chance of a space is in the car park down by the lake / theatre; that puts you right on the Borrowdale Road.