What is considered good vacancy duration and good tenant duration

[Cali Skier]

We are on our 3rd property and out 6th set of tenants.  Summary Question:  What is considered a good vacancy duration, and what is considered good tenant duration?

For us, the last two vacancies were 1 day and 1 month.  I think the 1 day is incredible and we got stellar tenants on that one.  The one month I think is good but I don't know what others are doing.  

Our longest tenant has been 3.5 years, Air Force.  Shortest tenant was 9 months, but we bought that property with them in it.  3 and a half years is good I think for Air Force tenants.  We own in a base town and like base tenants.

How would you complete these sentence:  

"You are doing good if you can get your houses rented in XXXXX days/months"  -assuming normal qualifying tenants

"You are doing good if your tenants stay for at least XXXXXXX months/years"

Thanks so much!