It lasts for 18 seconds, it seems to reflects back.....scratch that; it has a chance to reflect back any single target spell. I have not seen it work on AoE's but once it procs you have 18 seconds of 5% chance to reflect back the above type spells.
That means, Mirror I is 5%, Mirror II is 10%, Mirror III is 15% chance to reflect once you have the effect show in your buff slot; ie, it proc'd. 18 second duration for the effect. Also it seems to have no level restriction on spells it can reflect, only the type of spell it might reflect. I apologize for the repetatitve nature of this particular breakdown, but the repetative nature of the questions has given me the desire to do so <<sardonic grin>>