Find the right balance between your budget and your campaign's audience size.
With a small budget and a small audience, it will take time before frequency becomes a problem.
But, if you have a huge budget (thousands of dollars/day) you'll need a much larger audience to target. If your audience is too small people will quickly start seeing repeat ads and get annoyed.
If the frequency of your campaign goes above 5, it's time to start monitoring it more closely.
If the performance becomes too low either refresh the design of your ad or try a different value proposition.
If you're completely stuck, stop the campaign and target a completely new audience. Go back to the original audience after a month and your results should be a bit better.
Exclude people that have already converted as a result of your targeting.
If someone has already bought the product or service you're advertising, don't keep advertising the same product to them! The customer will get annoyed with your company and won't do business with you again.
Use a Custom Audience to exclude existing customers from your targeting.