There is no lag time in clustered ONTAP 8.1.x. This is available in 8.2:
::> snapmirror show  -fields lag-time 
source-path        destination-path              lag-time  
------------------ -----------------------------             --------- 
vs0:vs0_vol          vs0:vs0voldest                     135:55:33 
 
Thanks Sebastian!
I can't find a '-instance' option.  
We generally judge problems by the schedule x 2.  If the snapmirror is supposed to replicate every hour, if the lag is 2 hours or more, for example, fire off an alert.  
There is an 'is-healthy' in 'snapmirror-info', but it's a boolean option, it doesn't report how far behind from the source, the destination is.  The 7mode 'snapmirror-status' child_get_string("lag-time") is how I did this before.
-Blake