When would you buy a property with a negative cashflow?

[Mary Jay]

Are there any situations under which you would  buy a property with a negative cashflow? 

May be if you know the rents will go up in the future... Then how long would you be willing to wait till the rents catches up with the mortgage? One year? 2 years?

(If lets say you would buy a property with the negative cash flow but you dont worry about the cash flow, because you will be flipping the property, what margin would you need in order to buy it if you know you cant rent it out as a back out strategy)...

Thank you!