It is interesting that FBI director James Comey, who must know how many emails are involved, has not, as far as I know, released any precise numbers. (Or precise anything for that matter.) Let us say, for the sake of argument, that the number is 10,000. If you get 40 agents to go through them, that's 250 emails each. (There are more than 30,000 FBI agents so, to point out the obvious, if there are more emails you bring in more agents.)
Each agent searches for Clinton-related emails. The agents then divide their Clinton-related emails into two types. Trivial or Problematic. This might take a day, probably half a day. As some sources quoted here by Newsweek, suggest that none of the emails are either to or from Clinton, this might only take a few minutes.
But let's say some problematic emails exist. There are two ways in which they could be problematic. They are classified, or they contain remarks about Clinton's private server or the destruction of emails. First, an agent runs a search on the problematic emails to see if any of them are classified - the designations are pretty clear. And now it is the afternoon of the first day. You stay up late to determine whether you have any classified emails you've never seen before or if there are any emails suggesting Abedin or Clinton broke the law in other ways. If there are you hand them over to the Department of Justice right away and let it decide how to proceed.
If you're not sure about what you've got at the end of the day, or if you're the nervous type, as Comey clearly is, maybe you sleep on it. And the next morning you do what you need to do - or not do - based on the information you have discovered through investigation.
To make this vague premature announcement a few days before an election is either lazy or malicious and reduces the FBI to the FB, the Federal Blurt.