^WSS. It's the same as for a moses basket. The official advice is until 6 months or until baby can sit up (whichever is sooner). Some babies will grow out of them at a younger age than this though. Personally, if somebody was offering me a nice crib like that I would snatch their hand off! If it's free, you can buy a cot bed later on and you'll only have spent the same money, but by having a smaller crib first you save space in the room that can be used for a changing table or something. After baby can roll, I wouldn't use a changing table so then you can move the crib and the table out and get a nice big cotbed (some are absolutely massive!).
One thing to check with second hand furniture is how far apart the bars are. If you Google what the safe space is, make sure it falls within that. Some older cots aren't very safe so I'd want to check that and if it's been painted, make sure it's baby-safe paint and not flakey or anything. Also make sure you buy a new mattress!