The minimum depth for standard Smart-UPS Rails is 21.75"" and the maximum depth is 36"".
This applies to the SU032A (Used with most all SU,SUA,SMT,SUM, and SMX units) as well as the SURTRK and SURTRK2 (Used with Smart-UPS RT Products).
If your Smart-UPS is equipped with cleats (metal pieces attached to the sides of the Smart-UPS chassis that slide into the rails) the minimum depth is 29"". If you are using your APC rail kit to mount non-APC equipment your experience may vary.
Please note that some networking specific or custom enclosures may need to be adjusted to accommodate the depth of the rails.