In most applications, it is safe to remove the forms 48 hours after pouring concrete. This leaves the concrete still slightly malleable for smoothing the sides of the structure. In temperature extremes, it is best to leave the forms on for longer.
Concrete forms consists of lumber pieces and stakes that support freshly poured concrete and keep it in the appropriate shape until it has cured enough to stay in position without extra support. Double-headed nails connect form pieces for easy removal.
When cured at temperatures between 65 and 75 F, most concrete takes around three days to cure completely. Removing the forms after two days leaves just enough give in the concrete to round or otherwise smooth the areas covered by the forms. In colder weather, it takes longer for concrete to cure. If temperatures drop below freezing, the curing concrete needs to be heated to cure properly. In hot or dry weather, concrete cures more quickly and forms need to be removed earlier to allow for proper finishing.
One way to test to see if it is the right time to remove the forms is to press the concrete surface gently in an out-of-the-way location with a finger. If the concrete has a lot of give, leave the forms on longer. If the concrete feels solid but has a slight sponginess, then it is time to take off the forms.