I will respond to reports about 1/4th to half of the time. Most of the time if I don’t think any mod action is needed, I will not respond, because it’s usually about something small or just about general opinion about something (like whether or not something is regarded as an insult).
The majority of the reports we get are either about spam or moving a thread to another forum and those I never reply to, I just take the action (usually).
I think if you’ve sent a report and no action was taken, it’s probably a sure bet that the mod didn’t think any action had to be taken OR the mods are busy discussing the situation. Sometimes we talk among ourselves about a report, even if we don’t reply to the reporter.