I actually had to look this up because my suet blocks never last long enough to worry abt it going bad... but they say you can tell when suet goes bad because it will smell absolutely rancid! Typically around 2 weeks. :t: They advise that if your suet blocks don't last 2 weeks before going bad to put out quarter/half blocks and freeze the rest.
Maybe I need to make a change. My suet blocks still look brand new!
This morning I saw a Downy on the suet feeder; not sure if it was the first time for him, but it was the first time seeing him there. He was on it for maybe 10 seconds; took a few pecks, then went to the hulled sunflower seeds. If I don't see more action in the next week or so, I'll switch to peanut suet.