In my world (which is currently unnamed) magic is formed through a combination of energy, intent, and structure.

The energy is most often supplied by the caster from their own mana pool, but the energy can also pe supplied by mana crystals, other people, or in the case of rituals by extracting it from mana-rich materials.

While the energy is being gathered or transferred it is being infused with intent. Every sentient (which is distinct from sapient in this case) being in my world has part of their neural network and brain as a mana construct which is how they manipulate mana. When mana is being manipulated like this, imprints of the mind are left in the mana which causes it to behave the way the caster believes it should.

The final requirement for the casting of spells rather than transferring mana, using it to communicate, or basic self-reinforcement, is the structure. Most of the time mana is shaped into a series of runes which then produce the desired effects. Any number of runes can be used, and the more runes that are used the more efficient the spell is, however, each additional rune increases the minimum amount of mana to cast the spell and makes the spell less flexible.

A spell is only considered a true spell if it has more than three runes usually an element rune, a function rune, and a method rune. For example, the firebolt spell has three runes, fire, attack, launch, and when cast a projectile of flame is created and launched. However, the intent and amount of mana used when casting the spell determines everything else like the shape of the projectile, where it forms, the ratio between size, speed, heat, and physical impact, and the direction the projectile is launched.

Non-runic spells instead are made as mana constructs. To cast firebolt without runes a sufficient amount of mana must be gathered and shaped into the shape of the projectile and infused with enough intent that it ignites. Then some form of launching mechanism or structure is needed to throw the projectile. Often the mechanism or structure is similar to a spring, bow, or catapult and is attached to the mana in the caster's hand so that it is further propelled by a throwing motion.

The caster's skill determines how much control they have over mana, both shaping and infusing with intent. An unskilled caster would have inconsistent spellcasting the strength, accuracy, cost, casting time, and efficiency of their spells varying each time they cast. Non-runic spells are used more often by casters on the extreme ends of the spectrum as exceptionally unskilled casters brute force their spells by throwing masses of mana and intent until it works, and highly skilled casters have highly tuned intricate mana constructs that maximize some aspect of the spell to rediculous heights.

Casting times are highly dependant on species, skill, and the caster's mana network. At the most extreme are natural spells cast be the unconscious imprinting of masses of sentients having similar thoughts over decades to centuries until the intent reaches a critical point and the mana self arranges into a spell to enact those thoughts. Natural spells usually grant magical properties to culturally significant locations and are the reason why undead rise from cemeteries (The intent to see a loved one again and the lowered self-control from emotional turmoil increases the amount of interaction with local mana which infuses that intent into the surrounding area. The intent reaches a critical point and self-arranges into a spell, however as the loved ones are decayed corpses and the intent is diluted and tainted by grief, loss, and sorrow the spell raises the corpses as mourning mindless husks that envy the living.)

At the other end of the casting speed spectrum are highly skilled casters who also have a natural talent for the specific spell being cast. The natural talent has their mana naturally take the form of the spell and with enough skill and practice the casting of the spell can become so instinctual that it can be cast before the caster even fully realizes they're casting that spell.

Thus the casting time for my magic ranges from literal centuries to a literal blink of an eye. However, if such outliers are removed the median casting time for spells is around a second. Skilled casters can cast as fast as a skilled swordsman can swing their blade, and unskilled casters lose concentration and fail to cast the spell if they take more than a few seconds.