GarageBand for iPad only displays bars and beats in the TImeline and not min.sec.
The maximum length of a Song is 320 bars. To extend to that length, press the + button at the right end of the timeline to open the Song Sections popup window. Set it to 320 bars (if you are working in a single Song Section).
The final duration of your song is now determined by the tempo value of you Song.
Technically you could set the tempo to the lowest value to get the longest song duration (if you are not dependend on the beat).Hope that helps
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