How long in fridge for sushi steaks? I ordered some sushi grade steaks online.  They came frozen, as they were shipped with dry ice to keep them nice and cold.  Frozen, I cut them into pieces and put them back in the freezer, taking out one chunk at a time and thawing in the fridge.  

Sushi grade steaks do not need to be cooked, since they are kept frozen at temperatures where parasites cannot survive so there is no concern of the meats contaminating anything. 

Once thawed, how long do I have to keep them in the fridge until they pose a health risk?

Since this might depend on the type of fish, I will leave the question open to all fish types.  However FYI I ordered yellow fin tuna (Ahi), yellowtail tuna (Hamachi), and salmon (Sake).  Also Capelin roe (Masago), and salmon roe (Ikura).
I have always made sure that I only defrost enough to meet the needs of the moment for sushi.  If you cut the blocks into 4 to 8 rolls worth, they should defrost under running water in just fifteen to twenty minutes, just about the time it takes me to do a batch of sushi rice.  That said, fish once defrosted will start to lose flavor immediately but will remain edible for 2 days.  Once it starts to smell, get rid of it immediately, and I would play it very safe on that "starts to smell" the faintest whiff should be enough to send it to the bin.