Common audio signal voltage range on Line out connector
Usually consumer line outputs are nominal -10dBV (about .316V RMS). 

But many devices put out less than this, for example a Mac Pro 5,1 line out puts out about 180mV RMS (250mVp) with a sinusoidal input of -20dBFS and maxes out at about 2V RMS (2.8Vp) with an input of 0dBFS. These are open-circuit measurements I have taken myself.

So to be safe you should design for -10dBV nominal (max headroom would be 20dB above that so +10dBV [3.16V RMS]). That way most consumer devices won't clip your input.

But you should also choose a device you think might be used with your design frequently and measure it so you know for sure what you are expecting. Like a headphone jack on an iphone or something.