While there is not an exact period that it will take the mattress to expand completely, you can just wait for 24 hours. After the 24 hours, you can notice whether the mattress has retained its appropriate size and shape by just looking at it. The wait will be worth it for you to enjoy the experience you get from the mattress; whether you are sleeping on it during the day or at night after a long exhausting day at work.
Just looking at the mattress after the recommended 24 hours, you will be able to use the mattress in a much safer and simple way. At this point, the mattress will be fluffy and much comfortable for you to lay on. The first night may seem a little unusual but you will get to use the mattress in a much more comfortable manner.
You are probably eager to jump on the mattress and relax your back that has not rested on a soft type of mattress like a foam mattress, but you will have to wait a couple of hours more. While some manufacturers may recommend that you wait close to three days for the mattress to expand, it may not be exactly that. Others might even ask that you just wait close to 48 hours for the new mattress to expand completely before you start using it.
If you are not the patient type, you can speed up the expansion process of the mattress by placing the mattress in a warm environment. You can place the mattress over a bed in a room that is warm and well-ventilated. What you are subjecting to the mattress too is natural warmth and flow of air through the well-ventilated setup. This will be effective in making your mattress retain its normal size and get to its full potential within the shortest and most convenient period.