I think my maximum is about 2 hours. That was on a visit to a new independent bookstore I'd found out about, and they had some really interesting books I'd never seen anywhere else.
@albie - the reason I don't buy books from the internet is usually because I find it a very clinical way of choosing a book - I have to already know the book I want to buy and in a few clicks all I will usually see is the cover before deciding to buy it.
As a reader, I want to be able to see the book in its entirety, to feel the weight, look at the typeface, read the blurb, check the pages numbers and maybe even read a couple of paragraphs. I want to be excited at the prospect of reading an author I've never heard of, or finding a new book by a favourite author, or just being swayed by a beautiful cover, basically I love the serendipity of buying books in a shop.
On an internet bookshop, it's very rare to find a book by chance, and even if I do find one, I would rarely buy it without having looked at it in a shop first. For me, internet book shopping is saved for expensive books that I can get at a cheaper price, or a book I know I want to read that isn't stocked in the shop and would need to be ordered in.
My time is precious, but one of my favourite pastimes is browsing bookshops. The smell and feel of all those books, the atmosphere of calm and serenity, and that tingle of excitement of knowing I might just find the most wonderful book I'll ever read all add up to a heady experience that I wouldn't give up for the world.