Every tuesday at 10est/9c. They show it back to back, so if you missed it at 10 you can watch it at 11. Also yestardays episode will be shown on monday at 10 eastern if im not mistaken. The show is heating up slowly and it's getting really good. Too bad you can't catch it from the beginning becuase there are a lot of things that just wont fit in if you start watching it from now on. "24" just doesn't compare with Shield, no show does.
If someone is interested you should just buy season 1 on dvd and you won't be dissapointed. This is the 4th season so you have a lot of catching up to do.