You can return as many times as you wish up until the time Talana warns you about going back. When she warns you, that's your last chance to return to Broken Valley "as you know it."
I'd like to close by quoting Bellegar's famous last rhyme:
"Cursed be the mother that bred thee! The wench must have been a hell-spawned banshee!"
be warned that if you are playing on the xbox, if you leave the quest with Sassan from the time you resurrect her to the time she summons the demon it will cause the game to break. Sassan will be unable to summon the demon and you can only get round it by doing the whole quest again without leaving, so make sure you are ready to leave Broken Valley before you start that quest.
