You cannot get a timer in GarageBand.
Max recording time in GarageBand for iOS is 32 minutes, based on a song limit of 320 bars with BPM set to the minimum of 40 and there being 4 beats to the bar.
I always use iTalk Recorder for general audio recordings. You can record at a more appropriate bit rate for the content whereas GarageBand will always record at CD quality, making larger files.
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