Hello Gavin,

Transient model data cleans up automatically by scheduled cron job in Odoo.

Go to menu Settings >> Technical >> Automation >> Schedule Actions and you will able to see cron record for to clean up Transient model data. If not activated in your instance, please activate it. At specified duration, system will clean all your Transient data.

Technical Note :  Though name contains "osv_memory" it doesn't means it will not clean up latest styled "Transient" models in V8. System will clean all the V8 styled "Transient" models.

For proof, you can go to file /addons/base/ir/osv_memory_autovacuum.py and you will see following class,

class osv_memory_autovacuum(openerp.osv.osv.osv_memory):
    """ Expose the osv_memory.vacuum() method to the cron jobs mechanism. """
    _name = 'osv_memory.autovacuum'
    def power_on(self, cr, uid, context=None):
        for model in self.pool.models.values():
            if model.is_transient():
                model._transient_vacuum(cr, uid, force=True)
        return True

As per "Bold" lines on above code, system will vacuum all the Transient models too. 

Hope we made your confusion 100% clear now,