Time for some math:
Assuming that you're cold-crashing from around 70F to just above freezing, the volume of the gas in the head-space will shrink by about 7%, and of the beer by about 0.2%.
As a specific example, a 6G fermentor with 5G of beer will suck back ([6-5] x 0.07 + 5 x 0.002) = 0.08G, or about 2/3 pint.
So if you wanted to use your blow-off solution as a reservoir without sucking any liquid all the way back into the fermentor, you'd need at least 8 feet of 1/2" diameter tubing.
FWIW I've put together a little spreadsheet
here to calculate the total suck-back volume and the corresponding line-length for user-specified fermentor and beer volumes and blow-off tube diameter.