Speaker cable length

 With low output impedance ss amps and the typical heavy gauge speaker cables used today, the length is less critical than years ago when high output tube amps and thin gauge zip cord were used exclusively in yesteryear.
 Reasonable differing distances with most speaker cable won't affect time very much, but depending on the cables used might load the amps channels differently.
 Perhaps the best reason to buy identical lengths of speaker cable is to facilitate resale value.
 FWIW, I always use identical lengths.

If you are using 6 or 8 AWG wire and you are not running in excess of 30 - 40 feet, you will not feel any difference in the sound.

Make sure the cable lengths to both the speakers are the same, and do not coil loose cable if there is any.  Bend them and make them run parallel.

@cakyol. My new lengths are 15 and 20 feet of 10g cable. 

Simao,

you will be fine with a 10 awg wire for those distances. No worries. 

Here is what I just personally did. Your mileage may vary.

I had my 2 power amps on the side. Therefore, my cable runs were about 10 feet for the front speakers and about 18 feet for the second set.

I decided to place my power amps in the center on the floor instead. This allowed me to shorten my cables in the front to 5 feet and the sides down to about 13 feet.

I am listening right as I write this. I can hear much more detail in material I am quite familiar with.

Shorter is better. The shortest is best.

I was doing a guitar session once at the Record Plant with a bigtime producer. (name dropping not necessary). This was probably 40 years ago. I brought my whole rig in including my full pedal board of effects. Each effect had a very short cable in between.

The producer asked me, "what the F* is that?" I said it was my rig.

He said, "Get rid of it". I said, What??? That's my sound, that's my tone".

He said, " No it isn't. The tone is in your fingers and not in all that crap on the floor".  He told me that he wanted me to stand right next to my Marshall, and use the shortest possible cord possible. I did this. And guess what?TONE CITY. Les Paul. 6 foot quality guitar cable. Marshall. 

Since then, I have never used any effects. None. The tone was indeed in my fingers. I only use transmitters now live, and if I am in the studio,...

Yep! Les Paul. Quality 6 foot cable. Marshall. Battaging.

Quality of cable is just as important as length of cable. (to me)

Enjoy!