The maximum length of any one song in Logic is 2138.5 measures in 4/4. This is the same as 8211840 ticks per song. This number is a fixed limit in Logic.

Logic's internal resolution is 960 ticks-per-quarter (4 x 960 = 3840 ticks per 4/4 measure), which at 120 BPM results in a time of 71,28333 minutes or 1:11:17.
The formula is: Time[minutes] = 8211840 / 960 / BPM

To increase the maximum time you have to reduce BPM. A BPM of 60 will result in a maximum time of 2:22:34, BPM 30 gives 4:45:08.

Related to the above:
Q: Are there any disadvantages when reducing BPM?

A: What happens is that the number of MIDI ticks per time will be reduced. This means that the timing resolution for MIDI data in general (be it notes or controller data or MIDI based automation) will be reduced. Whether this is a problem or not depends on the song.