1) You do not "have to go" anywhere ... its fun tho, there are quests, and items, and stuff ... so why not?
And yes, its better to go there on higher level rather than lower, Underdark is presented as "the dangerous part of this world".
2) Depends on how much you shall explore it ... i usualy stay there aproximately twice as long as on surface, and now there was new location added ... so maybe it become thrice.
3) "Somewhere abowe 10" is the closest official answer we get ... even tho, there is countless theories around this question.
Hope it helps you.
