3 days gets them a check, probably investigation if they are researching in a library. That accounts for the time to find a book on the subject that they can actually read. Obviously finding out Revenant is easier than Revenants come back in other bodies and can't be stopped. I hadn't heard the 1 year limit, but I skim a lot of monster entries if they aren't major focuses.
5 days gets them a check with advantage.
7-10 days they probably just find everything available.
Now, this is assuming a city or temple library with a large selection of tomes and not terribly well organized beyond "undead monstrosities in the back, horrific beasts are second to the left". If you go to a temple of a knowledge god or goddess whose priests would be better organized, perhaps cut a day from each category.