What is the Optimum length for counter flow chiller?

[Oh Crap]

What is the optimum length to make a counter flow chiller? Or does it matter
I have
10.5 metres 10mm copper
10.5 meters qualpex 3/4"
Will I use 7 metres or  the full length (or is that too long)?
I'm also soldering wire around it for turbulence.

[DEMPSEY]

I don't think its a matter of how long because simple put the bigger it is the quicker it will work