Correct, it's only while using Sense Void, and it doesn't consume any of your spell slots. Really you aren't casting the spell at all, you're just perceiving the Void in your target and deducing their qualities and attributes, but the spellcasting mechanic seems to be the most straightforward method of representing this in the game. Compare to some of the transformational spells in Spell Compendium that let you use a small menu of low-level spells at while while you're transformed.