Question About Tank Cycling Time

[MMAX]

I have had my tank running since Feb 2/08. On the 4th I added live rock and on the 8th I put in 10 hermit crabs and 4 snails. I kept on testing for ammonia and the most I ever got was about 0.50 ppm. The last time I tested was the 24th and it came in as 0.25 with nitrites and nitrates at 0. Shouldn't the crabs, snails and live rock caused the ammonia to spike fairly high and why haven't the nitrites shown up yet?