I'm in the middle of this right now. I'm just trying to do a rate and term refi out of a HML. I have good income, good credit and good DTI. I don't need the rental income to qualify, though the property is rented. I'm finding many lenders want ONE YEAR seasoning before they will use a new appraisal. I was hoping it would be quicker, but at this point, I'm just hoping to find someone who will do it with six months seasoning.
Once you actually get a lender to OK the loan, plan on 4-6 weeks for the appraisal, to get the loan fully approved and go to closing.
So, overall, 8 months, minimum, is what you should plan for.
If there's a lender who will do it more quickly, I'd like to hear about it.