Refinancing: how soon can rents be used in valuation?

When purchasing a property (multifamily in my case), how long do I have to hold hard money loan (conventional is not an option) before I can refinance with a traditional lender using the income produced by the unit(s)? If the units were rented by previous owner as well, can that be used to show long-standing income history?

Also, as long as the property is producing well and has equity, will it still be difficult to obtain loan through LLC?