I recommend spending at least two weeks researching before you start a book. This is vitally important if you are writing historical novels or novels that take place in real world settings, with real life objects or ideas.
Not only is it important to get your history and geography right, but even simple concepts need to be researched.
Does a horse have hair or fur?
Do sharks eat dolphins?
Do penguins live in the North Pole?
When was velvet invented? Would my character have a velvet dress in the 12th century?
Do the Chinese believe in Santa Clause?
How long does it take for a body to decompose? (Good to know for mystery books.)
Where does baking soda come from?
How is bread yeast made?
Your characters also need to be researched:
Is this an appropriate accent for a character from South Africa?
Would a six-year-old normally use that word?
Does your character’s religious beliefs forbid them from wearing red?
Does their hair curl at the nap of their neck when it’s humid?
You can see how researching your material will save you from embarrassment later on. You can’t depend on your editor or agent to catch mistakes like this.
Research is the author’s responsibility.