K.M.Weiland, who is a font of writing advice, has a good interview on Writer.ly discussing when to know you're ready for an editor. I particularly like her idea of having "heavy hitter" critique partners and "nice" critique partners to use as buffers before moving on to getting an actual editor. If you're thinking of doing so, it might be interesting to read her methods.
Writer.ly Community – One Question Interview with Writer.ly
How do you know when you're ready to pass your work on to others?
Writer.ly Community – One Question Interview with Writer.ly
How do you know when you're ready to pass your work on to others?