The answer is "it depends".
A realtor can certainly give you a average "Days on Market" in your local area, but as an investor, you need to be more specific to get a meaningful and actionable "Days on Market".
Talk to a local realtor and be prepared to provide the following:
Which area (which zip code, or better, which subdivision)
What condition of houses are you considering?
What is the size of houses you are considering?
What is the age of houses you are considering?
Having access to your local MLS would certainly help. I recommend developing a good working relationship with a local investor-friendly realtor.