I cast a concrete lid to replace the broken one on my well pit. My lid is 30"x 36", and 2-1/2" thick and reinforced with rebar. I am keeping it covered with a tarp and misting it every few hours. How long before I can safely move the lid to my well pit?
As a teenager, I had a summer job pouring concrete to cast septic tanks and their lids. The lid of a septic tank is exactly what you're creating - we made them in circles about 5 feet across and three or four inches thick with rebar inside and three rebar hooks for lifting.
Each morning, we would use a front-end loader (tractor with a bucket on front) to pick up the lids we had cast the previous day. These would be set aside to cure for another few days before installation. Then, by the afternoon, we would have the forms set up for pouring again. The concrete arrives at 1pm, we pour by 2pm, and then mist the tops to keep them from cracking until sundown (Texas heat). Like I said above, by morning, the lids are able to be moved and we start the process all over again. Based on this, I would say that the lid can support its own weight after 24 hours, and will fully harden over the course of a few days.
To give yourself a safety factor, double those numbers, so go with this: Wait 2 days to move it at all, and wait 5 to 7 days before putting it into place.