What is a safe upper limit of audience saturation in a Facebook ad set?

IMO it's important to distinguish between audience saturation and creative saturation for this assessment. 

-> Creative saturation -> typically this happens when your creative is seen by everyone in the audience - and creative performance starts to deteriorate(typically that means worse CTRs, CPAs, ROAS-es). I typically find that this starts to happen around a frequency of 1.8 - 2 -> but really I'm looking at CTRs, CPAs, ROAS-es to ascertain creative saturation.

You typically remedy this by adding new creatives - could be either:
        -> variants of current winners(videos with diff thumbnails, display ads with diff backgrounds etc.)

        -> completely new ad creatives.

-> Audience saturation -> You see this when you ad set level performance does not improve even after you implement creative refreshes to combat creative saturation. In my experience this starts to happen at a first-time-impression ratio of under 50% on a sustained basis -> although again much like creative saturation I'm looking at performance metrics to see if they continue to stay down after I implement creative refreshes.

If I'm seeing sustained audience saturation, then I typically start expanding to or testing new audiences -> this Quantmar thread should offer some inspiration: https://quantmar.com/434/What-best-examples-facebook-lookalikes-that-work-campaigns