Very nice shot TP.
DP it depends on the focal length of your lense. Roughly near 0 Dec T=700/FL so for a 50mm lens you get about 14 sec. Close to the pole roughtly T=1000/FL so again for a 50mm lense, 20 sec. Near the pole with TD's 10, theoretically 100sec. Though you have to take into consideration how close the edge of the field will be to 0 dec and adjust accordingly