Without a repeater, a sub-rate ckt can not have anymore that 18kft of copper. So, depending on how the ckt was delivered to the d-mark, that would determine how far past the d-mark you can extend it. If the ckt was delivered using IDSL technology (card placed in a ncte mounting at the d-mark) you have 18kft past the card to work with. If it was delivered the traditional way, over straight copper, you will just have to try it. And remember, sub-rate is terminated as an RJ48s, your pinouts are 1&2, 7&8.

(Edited due to auto-correct. Would not have been good if left)