When you cast a spell there is a spell action that takes time. Now, that time may be 10 seconds or it may be 10 minutes but there is not a time when a spellcast ACTION will take more than an hour. Even that long of a time will be including raising of energies for the actual action of the spell. So when you see "cast for 20 hours" beware. Casting over a period of time like that consecutively is ridiculous to claim. You literally would have to remain in your raised state of energy for that whole time for it to be effective and that would be without eating, drinking, or using facilities. Does that sound good to you? No. The statement could be pertaining to leaving candles burning for the duration of a spell conclusion, and that could take hours, but the casting ACTION (which is where the power of the spell sits) would be far less of a time. The spell releases all at once (within the ACTION), not over a period of time like that. So, the spell could take hours, but the casting (which is the action part of the spell) will only be a fraction of that time.
Now...casting over a period of days or cycles is something different. This is when you take a spell and repeat it over a period of time. It's not one long, drawn out spell. It's the same spell (maybe a few daily modifications) each day over a period of days, weeks, or months. But again, the casting (the ACTION) part of the spell only takes a small fraction of time.
Why did I feel the need to share this? Because of claims I've seen in browsing some of the newbie-witch groups. It's crazy and stupid and you can tell within that statement that there is fantastical aspects to that person's claims.
So please realize that the strongest of witches will focus on casting the energies within the shortest time to allow for the 'burst' to be accepted into universal flow (for lack of better analogy).
Okay....onto your regular scrolling now. :)