Igneous rocks can form in a variety of timeframes depending on their type and the processes involved in their formation. Generally, they are classified into two categories: intrusive (or plutonic) and extrusive (or volcanic).
Intrusive igneous rocks, like granite, form when magma cools slowly beneath the Earth’s surface. This process can take thousands to millions of years, allowing large crystals to develop, creating the coarse texture characteristic of these rocks.
On the other hand, extrusive igneous rocks, such as basalt, form when lava erupts onto the surface and cools quickly. This rapid cooling can occur in just a matter of days or even hours, resulting in the fine texture of these rocks.
So, the time it takes for igneous rocks to form can range from just a few moments in the case of extrusive formations to many thousands or millions of years for intrusive varieties, depending on the cooling rate of the magma or lava.