Also, I'm not able to find back the discussion, but I think a good number of people have a houserule in which Bladeturn x and the like have a duration, and may block up to x attacks during that duration. IIRC, most people have them last one round per caster's level.
As far as I'm concerned, duration is one minute per caster's level but such a spell "loses" one use each minute. For instance, a Mass bladeturn cast by a 25th level has twenty-five "uses" and lasts twenty-five minutes or until it uses all of its twenty-five "uses". Now, let's consider that, fifteen minutes after it was cast, the caster enters a fight that lasts three rounds and during which he "uses" five uses. Then, the spell only has five "uses" left: 25 (initial) - 15 (fifteen minutes) - 5 (uses during fight). So, if the spellcaster isn't attacked any longer, after five more minutes, the spell dissipates, having last twenty minutes (and three rounds) instead of twenty-five.