How much weight does a long term tenants have on yield?

[Will Murphey]

I have 2 rental properties that have been rented by the same tenants for over 2 years. One set of tenants have been there for 33 months the other home/tenants for 28 months. I want to sell these two homes and I am trying to figure out what return I should offer. Specifically, I am wondering does the length of the tenants stay play into the investor mind? Would these 2.5 years add anything to the ROI ask? Maybe 1/2 percent.

They yield 25K annual after expenses with property management in place.  Would 1/2 percent be fair for the term of the tenants stay or is this not even a factor investors consider?