Does anyone know how long these last? I brought some colour coded ones when we moved into our current house 3 years ago. I recently purchased another pack and the difference when cleaning is amazing the new cloths just pick up so much in comparison to my older ones. Same brand btw no difference in the cloths other than age. I'm loathed to throw the old ones out (I have a fair few 
!). I've always washed them without conditioner etc so I thought they would last longer; they are in ok condition but clearly not picking up like a new cloth. So how long should I hope to get the best out of them?