Hey hun,
There's no set time, all babies are different. Mine wasn't a long feeder, but other mums have babies pinned to them for 23 hours out of 24. Its not really comparable.
Remember that Bethany needs to drain one breast fully before switching to the other - some babies, mine included, only needed one side and never fed from both in one sitting.
However, unless Bethany is showing obvious signs of hunger after a feed, I would assume she's getting plenty, particularly if she's going into a contented sleep afterwards. Keep an eye out for lots of wet and dirty nappies and enjoy your baby!
Valentine Xxx