mine lasts several weeks with full access and I currently have uh 15 birds maybe, in that pen. They don’t free range once the rain and snow start. They stay in the coop or run. They still ate their feed and seemed to do fine over winter. I think I bought a total of 2 blocks last winter, they don’t get them the rest of the year.I personally like to control treat intake so I would not give them access to a block daily. I do use flock blocks but generally as a boredom buster for days when the weather is so bad that they don't want to go out and scratch around.
I've never bought a Flock Block since I've had my own Chickens. Over 2 1/2 years. Reason, it's basically Scratch Grains molded together. It's got 9% Protein, about half of what my Golden Comets need to prevent Feather Picking.
I prefer to limit the amount of Scratch Grains my girls get. If I bought one I would let them have at it an hour or two daily, depending on how many chickens I had.
I would offer it mid morning after they've had their fill of feed. GC