Here’s my basic understanding of how igneous crystallization works:

The pressure and temperature pieces of the formula are relative. The temperature at which crystals of a given mineral will form varies according to pressure, so that with increasing pressure the crystallization temp rises. So feldspar crystals can form at a wide range of temperatures depending on how much pressure the magma is under.

Then feldspars vary in composition, depending on whether they contain calcium, potassium or sodium in addition to the basic framework of aluminum, and silica. These different compositions affect the crystallization temp as well, so that there are high-temperature feldspars and low-temperature feldspars. At 1 bar of pressure, the range is from 600–1000 C.

The presence or absence of water is another variable affecting the crystallization temperatures.

The time necessary to form crystals from magma is not long at all. In fact, to get magma to solidify without crystallizing requires that it be cooled almost instantaneously, as when magma erupts to the surface or flows into water. This is what volcanic glass (obsidian) is. But the time of cooling will affect the size of the crystals that form from the melt; slow cooling allows for larger crystals to grow.

So yes, there are too many variables to give specifics.

I’ve been talking about igneous crystallization, because you mentioned feldspar, but crystallization from an aqueous solution, as for gypsum crystals or rock salt, is a very different process. When you talk about seeing crystals form over weeks, I assume you mean this kind of crystallization.