Go to jko and filter the search tab by "atrrs dl points", find the longest ones and start crunching. One that i recommend to Soldiers is field sanitation phase 1 which is in atrrs and is a 40 hr block on blackboard.
Special Ops Medical Prep Course is 144 hours. I registered in Attrs for that one. Has no pre-reqs, hopefully they have it for this fiscal year. If not, search course code 555 for all DL courses. Should show how many hours each course has.