IMC, it's a Knowledge skill check. I have a one-hour library search grant a +2 bonus to the check, or higher if the library is very large or especially well stocked. One or two assistants may use Aid Another to help in this check, as long as they have ranks in the same skill or in Profession (research).
Taking 20 on the check is allowed, but that requires 20 man-hours of searching. E.g., if the PC has two assistants, he can take 20 after a 7-hour work day.
If taking 20 is not enough to reach the target DC, but the DM decides the answer is somewhere in the library, add another +1 bonus per man-hour until the answer is found. That means you can happen upon the right book if you just keep searching long enough. But it also means that if the answer really isn't to be found here, you can waste an awful lot of time before convincing yourself of that.