I found this listing for this counter below.... It appears that the thresholds are in ms based on the values and the information below.
MSExchange Search Indices(*)\ Average Document Indexing Time
Shows the average, in ms, of how long it takes to index documents.
Should be less than 30 seconds at all time.
from the URL below...
Hopefully this will help answer your question.
Many thanks for getting back to me.