In 3.x it lasts as long as you can concentrate on it (you can't take actions while concentrating on a spell), so I'd go with that.1. Does Detect Magic have a duration? If so, what is it?
Yup. Also if three stirges are attached, it's a quick kill (though you might rule that a character can't have more than one stirge attached to him, for reasons of physical plausibility).2. How does the Stirge "blood drain" actually work? As written, does it mean death in three rounds if the stirge is not dislodged?