Skin temperature (also "surface" temperature) will be displayed as soon as you initialise CORE and is updated every 2 seconds.
When you first put CORE on it requires about 2 minutes to calculate your current core temperature (see this link to understand the difference).
As long as you are wearing CORE correctly, the measurement will update every minute with your real-time core body temperature.
If CORE receives invalid data e.g. it is worn incorrectly or not on the body at all, it will take longer before it will deliver a valid core temperature after you start wearing CORE correctly.