I laid it out last night intending to use it for NFSC this morning. Life got in the way -- how long is too long to let the butter sit at room temp? Thanks a bunch!

It'll be fine if you have only had it out overnight. Not sure of the exact days that it stays fresh outside the fridge though.

Salted butter is just fine for surprisingly long periods of time in a cool house. Unsalted butter, which contains no salt (a preservative), is fine overnight. Others may say it is fine unrefrigerated even longer.

Not sure what the official rule is, but when i was a kid, my grandmother never refrigerated butter. And she lived to 92!!

I know that mold will start growing on it if you leave it out for too long (just don't know what the time frame is though).
I've started buying the land o lakes spreadable butter because I can keep it in the fridge and use it to spread on food items like bagels. For baking I use the reg butter and sometimes take it out the night before as well.

Also wanted to mention that this weekend I needed room temp butter to use in my SMBC and only had frozen butter. I didn't want to wait for it to reach room temp so I grated it so I could use it right away. I've also read that you can pound butter (like with a rolling pin) to get it to reach room temp faster but I haven't tried this method yet. One of these days I'm sure I will.