When the crate drops initially got turned on, I got one upon log in for a few days. It then stopped for a few days, came back on, and is currently not doing so. I've heard that you get one every five hours, but I'm been on the server multiple times for longer periods than 5 hours and I've only had it drop once. Was curious about others' experiences and opinions with it.
Pretty sure it's random based on several factors, and the 5 hour thing is made up. I usually get around 1 every day or 2. I.e. looking at it now I have 3 and probs checked it 2-3 days ago.
There's an overall "cap" of crates you can receive. If due to RNG you get lucky and play enough, it's not too hard to reach the cap.