When the tune on which a ringtone is based is in the Public Domain, then we can make it as long as we like subject to it fitting in a mobile phone.

Unfortunately, when the tune on which a ringtone is base is not in the public domain, then we have to make it 30 seconds or less to comply with the terms of the MCPS/PRS rules for licensed works being used as ringtones.
IE The MCPS/PRS definition of a "Ringtone", as opposed to a "Whole Tune" is that a ringtone is less than 30 seconds in length.