Average lines are built to about 80-85 hours. If you're trying to fly min hours then you're going to fly more than 18 days(max days on unless you waive it). Junior pilots don't have access to the trips that are required to make a line min hours. The junior trips are the 3-day west coast trips. These trips are around 11-12 hours each. 18 days of flying these trips would only get you up to approximately 70 hours. You would actually have to waive days off in order to get a line. You will have to get some seniority to get some of the higher credit trips mixed into your line just to fly ~75 in 18 days.

As a junior pilot, you either waive days off (up to 3) to get a bid line or you go on 18 days of reserve.

If you want min days on, that takes even more seniority because you need a line with mostly higher credit trips to start bringing down your days on.