sometimes, it takes days before the mods take action on a reported spam post
If there is an obvious example of a spam post, it is impossible for the report to be processed only after a few days. There can be reports that are not clear for different reasons and are awaiting for a moderator to make a deeper research before acting, but obvious cases are processed in hours maximum. 
This (in other words) is to say that the moderator of a board is always online 24 hours a day and 7 days in a week?
well, hard to believe since from what I've seen here, most boards have just one user moderating that baord, and common sense tells me that when ever a post or comment in or on a particular board is reported by other user(s), the report appears only to the moderator of that board, and there is no way one person can be online 24/7, even if they appear to be online through their forum status.
Except reported posts or comments are broadcasted generally to all the forum moderators, regardless of which board the reported post or comment was made or posted, then I will agree that yeah, such report can indeed be treated within hours.
Anyways, I am just thinking out loud, no big deal regardless.