lifespan of hermit crabs

[h2o]

is there a general rule how long they live? seven month ago i bought the package from swf.com, 20 blue and 20 scarlet, with extra shells, now i am down to two scarlet and about 4 blue, and a bunch of shells (large and small). my astrea snails live longer, still have 17 of them. but they don't clean grass algea on live rock, i am afraid soon my grass algea will overtake my live rock. might need to buy hermits again.

 

[burnnspy]

The blues prolly ate each other and the scarlets, Ive had my scarlets for almost 2yrs.
BurnNSpy

 

[bogie man]

I agree with burn-n-spy. They most likely ate one another. I have only one hermit crab, who is about as big as an egg. I am always on the look out for bigger shells. He is starting to knock rocks over. I have had him almost 2 years. He's a one man cleaning crew.