The cabling standards provide excellent guidance on this subject. You are encouraged to purchase these online at either https://global.ihs.com or http://www.iso.org.

ANSI/TIA-568-C.2 Annex G
Cables may be installed in return air plenums, in ceiling spaces, riser shafts and non air-conditioned buildings such as warehouses and manufacturing plants where the temperature can be significantly higher than 20 oC. In order to ensure compliance with the channel insertion loss specified in clause 6.2.7, the horizontal cable distance may need to be reduced below 90 meters depending upon the average temperature of the environment over the length of the cable, the insertion loss margin of the installed cabling, and the insertion loss temperature coefficient of the cable.

ISO/IEC 11801:2011 Table 32 – Horizontal channel length equations
For operating temperatures above 20 oC, H should be reduced by 0,2 % per oC for screened cables; 0,4 % per oC (20 oC to 40 oC) and 0,6 % per oC (>40 oC to 60 oC) for unscreened cables. Where H is length.

This ANSI/TIA-568-C.2 standard goes on to provide additional guidance in the form of a table:

Fluke Networks does not allow you to adjust the Insertion Loss in any of its field testers based on temperature.Some field testers allow you to adjust the insertion loss PASS/FAIL line based on temperature. This is not permitted per ANSI/TIA, ISO/IEC and IEEE. If your cable is to be subjected to an elevated temperature, your design must take this into account by reducing the length of the link, not by relaxing the Insertion Loss limit.