If you divide these 12.000 grenades by 32 howitzers and 6 hours, you will arrive a roughly 60 grenades per hour and piece. Thus every minute one grenade was fired by every howitzer. Scharnhorst states that howitzers could fire 1,5 rounds per rminute (quotation in the Napoleon Online thread I posted before), so one grenade per minute seems realisitc.
If permanently 10 grenades were in the air, this means that - if all the 32 howitzers fired permanently - the average time of flight of a grenade was 20 seconds until it hit its target. This also corresponds with Scharnhorst's data concerning the length and burning time of the fuse within the grenades (same thread)
So Savary's statements seem reasonable.