Asumming you get surrounded by monsters , which are 8 monster in total for 0,5 hit/second per monster you will get 4 hits per second.
With this data you will last 187,5 seconds with each necklace but you have to think that between each raid you last some seconds without being hitted so around 187 seconds + 10 seconds between each raid = 197~ seconds for each necklace more or less.
This depends on the way you hunt and the amounts of monster you blocking.