When leaving butter out at room temperature to soften it's usually safe to leave your butter out for up to 2 days.
If you leave your butter out longer than a few days then you risk it getting spoiled and growing bacteria.
Also if there's any flies around in your house then make sure to cover the butter up as the flies can lay maggots into the butter.
That has happened to me before.
I had a stick of butter on the counter and was making toast with it and forget and left it out overnight.
The next morning I found maggots in the butter.
If the maggots had not gotten into the butter it would have been safe to eat.
Just make sure to cover the butter and don't leave sticks of butter out on the counter without them being in a container as the maggots can get through the wrap.