There is no time limit to what you can report.

I would suggest, however, that you consider why you are reporting these posts, and if it is worth the moderators' time, which after all, is limited. If it is a serious offense such as linking to malware or plagiarism, then by all means it should be reported. If it is simply because there are some spam posts, then reporting them won't achieve very much and will take moderators away from other more pressing reports. If the whole thread is spam, then you could also considering reporting the first post in the thread suggesting that the thread should be locked.

If I'm reporting posts for spam, I rarely go back more than a week. The spam exists because spammers get paid on a weekly basis for it. Once the week is past, the moderators usually have better things to spend their time on. There is no shortage of recent spam to report that means you need to go hunting for months old spam.