sps, at what point do you give up and throw it away?

[burgerchow]

Hello everybody, I've been keeping lps for decades, but just started sps this year. My question is when is an acropora totally dead. I've got some frags that I bought from j and l about 4 mos ago. Most are o.k., but got this big acro.( about 5 inches tall, 3 inches wide) that I think is dead. Haven't seen any polyps for months. I've been hoping it would recover, but it has'nt changed or grown in months. ( my green birdsnest and green millipora have doubled in size in the last 4 mos)
How long does everybody keep trying to save a frag? I guess as long as you can still see some living tissue, it might have a chance to come back, or am I wrong? Can sps actually recover? When do you pull the plug?