You're in the US now? Did you return after you got the background check? Because if so, that may have invalidated it. I've heard of people getting the report and then coming down within a matter of days or weeks and not having issues, but going back after the report's date and before applying could mean that you have to do it again.
If you want to be safe, I would go ahead and reapply. You can try to initiate the application when you come down with the original report... if they accept it, great, you don't have to bother with the apostille... but if they don't, at least your second report will be in process.
90 days is what I heard as well, but never found anything official.