For Thunderbolt 3 systems a longer cable can be used, but there are some important factors to consider. The longest passive Thunderbolt 3 cable rated for 40Gbps (required for dual displays via TBT3) at this time is already included and is 0.8m in length. Longer cables will need to be active cables. We don't sell any active cables but they are available at up to two meters.
For users with USB-C only systems a Thunderbolt 3 cable is not required. Instead, a longer USB 3.1 Gen 2 cable rated for 10Gbps can be used. We recommend no longer than 2m in length. However, due to the varying quality of USB-C cables on the market, a better alternative would be a 2m 20Gbps passive Thunderbolt 3 cable like our TBT3-20G2M: http://plugable.com/products/tbt3-20g2m
Please note: For docks with 60W (watts) Power Delivery charging capabilities a cable that supports 3A (amps) minimum is required. For docks that support greater than 60W (65W, 85W, 96W, 100W) charging, a 5A cable is required.