Infusing a spirit at home usually goes wrong in the same direction: the result is bitter, astringent or vegetal. The cause is time, and the recipes that suggest weeks are the reason.
Different compounds extract at different rates
Alcohol is a strong solvent, and the light aromatic compounds that give an infusion its character dissolve within hours.
Tannins, resins and bitter alkaloids sit deeper in the plant material and extract far more slowly, over days rather than hours.
An infusion left long enough to extract everything has extracted the harsh fraction along with the pleasant one.
Delicate material needs very short contact
Fresh herbs, citrus peel and floral material give up their aromatics almost immediately and turn vegetal or bitter within a day.
Peel is the clearest case, since the oils come out in a few hours while the white pith contributes only bitterness thereafter.
Removing the pith before infusing and straining the same evening avoids nearly all of the usual failures.
Dense material behaves in the opposite way
Dried spices, roots and barks release slowly and can sit for days without problems, though the strongest ones will dominate long before that.
Cinnamon, clove and star anise are potent enough that a small quantity for a short time produces more balance than a large quantity for longer.
Infusing each ingredient separately and blending the results afterwards gives control that a combined infusion cannot, since each has its own correct duration.
Strength and temperature change the rate
Higher-proof spirit extracts faster and pulls out a broader range of compounds, including the bitter ones, so a strong base needs a shorter infusion.
Warmth accelerates everything, which is why an infusion left in a sunny window behaves quite differently from one in a cupboard.
Cold infusion takes longer and produces a cleaner result, which is the trade-off behind most of the slower methods.
Tasting is the only reliable instruction
The correct duration depends on the material, its surface area, the spirit's strength and the ambient temperature, none of which a written recipe knows.
Tasting on a schedule and straining at the point it tastes right is the method every published time is a rough approximation of.
An infusion strained slightly early can be left to rest and will round out, while one strained late cannot be repaired at all.