How seriously does cable length affect USB 3.0 transfer speeds?

The USB 3.1 specification contains a table of wire thickness versus length:

11.4.7 Wire Gauge Table

Table 11-3 is a table of VBUS/Gnd wire gauges showing the relationship between gauge and maximum length in order to achieve the previously cited voltage drop values. The user should note that these lengths are the maximum length possible to meet the voltage drop budget, thus gauges smaller and lengths greater than the table values will fail to deliver the expected voltage value.

Table 11-3. VBUS /Gnd Wire Gauge vs. Maximum Length

American Stranded Wire Gauge │ Ohms Per 100 │ Meters Maximum Cable Length
(AWG) on VBUS /Gnd           │ (Maximum)    │ (Meters)
   28                        │   23.20      │   0.8
   26                        │   14.60      │   1.3
   24                        │    9.09      │   2.0
   22                        │    5.74      │   3.0
   20                        │    3.58      │   5.3

It also states:

5.5.7 Cable Assembly Length

This specification does not specify cable assembly lengths. A USB 3.1 cable assembly may be of any length as long as it meets all the requirements defined in this specification. The cable assembly voltage drop budget defined in Section 11.4.2 and the cable assembly loss budget defined in Section 5.6.1.3.2, limit the cable assembly length.

USB 3.0 boasts 4.8 Gbits/s as long as your cable is within the (approximate) 3 meter threshold. Anything longer and the full speed is not guaranteed. The length limitation is a small step down from USB 2.0's 5 meters.

For greater distances there are USB hubs or special signal extender cable. (Section: What improvements are made to USB 3.0 cables & hubs?)