Distribute items based on the time duration
Nice implementation, it's well structured and easy to understand. Few general suggestions:

int instead of long: the constants MORNING_SPAN and AFTERNOON_SPAN contain minutes in a day, using an int is enough.
Input validation: the talk duration needs to be validated. If the talk duration is greater than MORNING_SPAN or AFTERNOON_SPAN it won't be included in any track. If it's negative it will also lead to issues. In the Conference class ensure that the list talks is not empty.
Creating the schedule: I see that the schedule is created (lazily) in the displaySchedule method of Conference. I think that creating the schedule is an important action, it can be the responsibility of another class, like ScheduleMaker. Then, the list of tracks can be added to Conference in the constructor.
Talk interface: talk.duration returns a string like &quot;30min&quot;, while talk.duration() returns 30 as a long. I find it a bit confusing. If you need both, consider creating two different variables and two getters. Otherwise, parse the duration in the constructor and use only the long value.

Bin packing problem
Let's consider this example:
morning_span = 2m (2 minutes)
afternoon_span = 1m
daily_span = morning_span + afternoon_span = 3m

talk1 = 2m
talk2 = 2m
talk3 = 2m
total_talks_duration = 6m

Calling conference.makeSchedule() will result in:
numberOfTracks = total_talks_duration / daily_span = 6 / 3 = 2

Schedule:
Track1 = talk1 (morning)
Track2 = talk2 (morning)
talk3 ??

As you can see the current makeSchedule will create two tracks, but talk3 won't be included.
This bug can be fixed by running the other loop till all the talks are placed:
private void makeSchedule() {
    while (talks.size() &gt; 0) {
        // Create a new track
        // Add in the track as many talks as possible
        // Remove talks added to the current track
    }
}

Now all talks will be placed in the tracks, however, there are cases where the algorithm will create more tracks than needed. This is considered a Bin packing problem. Basically, fit the talks in the minimum number of tracks. Interestingly, my university professor developed an exact algorithm for this problem (pdf). I'll leave it to you for additional study.