When you can buy more coins for fiat price of the miner than than miner will ever mine back (after deducting expense's), that's when. Obviously if the reverse is true you should buy as many miners as you can afford.
Whenever I have bought miners I buy them to mine for 3-4 months in the winter for extra heat and then I sell them. Hopefully they have new hardware this winter to heat my home. The amount of money spent on the miner was near zero dollars because there is always some demand for used miners.