Arcane Magical Writing
To decipher an arcane magical writing (such as a single spell in written form in another’s spellbook or on a scroll), a character must make a Spellcraft check (DC 20 + the spell’s level). If the skill check fails, the character cannot attempt to read that particular spell again until the next day. A read magic spell automatically deciphers a magical writing without a skill check. If the person who created the magical writing is on hand to help the reader, success is also automatic.
Once a character deciphers a particular magical writing, she does not need to decipher it again. Deciphering a magical writing allows the reader to identify the spell and gives some idea of its effects (as explained in the spell description). If the magical writing was a scroll and the reader can cast arcane spells, she can attempt to use the scroll.
15 + spell level = Learn a spell from a spellbook or scroll (wizard only). No retry for that spell until you gain at least 1 rank in Spellcraft (even if you find another source to try to learn the spell from). Requires 8 hours.
This seems like a direct case of the rules conflicting with each other. However, upon further examination, we find:
Adding Spells to a Wizard’s Spellbook
No matter what the spell’s source, the wizard must first decipher the magical writing...
Next, she must spend a day studying the spell. At the end of the day, she must make a Spellcraft check (DC 15 + spell’s level).
Here we can see that Deciphering and Studying are explicitly two different actions. What the Spellcraft Check entry does is clarify the study duration indicated under Adding Spells to a Wizard's Spellbook, telling us "a day" of study is equal to 8hrs.
If the check succeeds, the wizard understands the spell and can copy it into her spellbook (see Writing a New Spell into a Spellbook, below).
Let's assume success.
Writing a New Spell into a Spellbook
The process takes 24 hours, regardless of the spell’s level.
Here we have a set amount of hours to go by. Writing a spell takes a full 24hrs of writing.
However, is your Wizard really going to write for 24hrs straight? Probably not. If we go by the guideline set out above under studying, your Wizard is likely to spend 8hrs each day writing, taking 3 days in total to complete her work.
How It Works
1. Decipher Arcane Magical Writing - Immediate. Complete a Spellcraft Check [DC 20 + Spell Level]
2. Study/Learn Spell (Wizard Only) - 8hrs, then complete a Spellcraft Check [DC 15 + Spell Level]
3. Copy Spell into Spellbook - 24hrs. No Skill check.
The total time dedicated to the spell would be 32hrs, and would probably take an average of 4 days to complete. At a minimum, it would probably take you 2 days (16hrs of work each day, 8hrs of sleep).