When do you need a moderator?

Your groups/forums are growing quite nicely and your user base is building, but there comes a point where you need a helping hand. This is where moderators step in. Moderators are members of the community with more privileges than a normal registered user, but without the ultimate control that an administrator possess. Moderators can undertake important forum tasks, such as locking, deleting or moving topics, issuing warnings or even issuing bans. They enforce the rules of the forum, and are essentially on the front line.

Inspired by The Super Moderator Guide here are some points on when to consider  taking a moderator on board?

If your forum is subject to frequent spamming, or attracts immature visitors
Don't ignore it - it won't go away, and don't think you can handle it all yourself. You'll soon see that people who post inappropriate topics or ads won't stop until they're challenged. And if they see that you're understaffed, they'll take advantage of it and find their own ways.
If your forum receives too many posts for you to read yourself
Again, don't fool yourself with this. When you start to feel overwhelmed by the number of posts that need reviewing, seek help. If you choose not to, then your guidelines will be broken without consequence, and you'll start to wonder why the group or your forum is losing activity. It's also a great feeling to go on vacation and know that the group will be well looked after while you're away.
When you feel you need a team to help you take your community to the next level
Assuming you accomplish all the criteria above, but your membership isn't growing and your group is starting to stagnate. This would be a great opportunity to make changes within your. Your members will enjoy the change and they will become curious again, as it shows that you're still looking ahead. Recruiting a moderator is a good chance to get advice from someone who's on the "outside". The moderator can supply some fresh ideas to make your forum more exciting and worthwhile for members.