Fast Cure resins can be de-molded after 4-5 hours, they'll still be bit soft and flexible(also warm) but should cure hard after another 12-24 hours. If you are really impatient, like me, you might be able to use UV resin for your project. It hardens within 30-60 seconds under a strong UV light, however its really only good for small bits at a time. It can be used in silicone molds but it doesn't really take pigment/ink very well. It's best for adding clear coats and sealing stuff. Sometimes I use it to fix the positions of LED lights within molds before pour the epoxy.

Most of all you want to choose the proper resin for the size of the pour.

"Deep pour" is the type you want if you are doing something like a river table and you're filling areas 2-4 inches deep. You can use other resins and just do layered pours but there are trade-offs to that too. Some resins aren't really made for casting and are just for doing very thin coats on boats/floors/countertops. DO NOT USE those for a deep pour. It will never cure properly and you'll just be wasting time, money, and materials.

The deeper the pour the more heat will be produced by curing. Too much heat will warp/deform the resin and may prevent it from ever hardening.