Rule 1, the manual is always outdated or plain wrong. In this case it is just inaccurate ;-)
The GUI really lacks information about expected rate of fire and length of mission (in rounds or minutes) so all you can do is painfull testing.
I did some, feel free to add to the table.
Veteran spotters and guns, flat map.
For batteries, one tube only, more tubes fire simultaneously.
Rate of fire is 1 round per x seconds.
Duration is in rounds, or minutes if m suffix.
US 60mm M2 mortar onmap
Mission: Harass | Light | Medium | Heavy
Rate of fire: 18 | 6 | 3 | 2
Duration: Q S M | Q S M | Q S M | Q S M
Rounds: 6 7 7m | 3 20 30 | 3 19 24 | 3 16
US 81mm M1 mortar onmap
Mission: Harass | Light | Medium | Heavy
Rate of fire: 18 | 6 | 3 | 2
Duration: Q S M | Q S M | Q S M | Q S M
Rounds: 5 10 15 | 6 10 28 | 3 10 25 | 6 12 22+
US 105mm M2A1 Howitzer
Mission: Harass | Light | Medium | Heavy
Rate of fire: 46-48 | 28-32 | 16-19 | 10-12
Duration: Q S M | Q S M | Q S M | Q S M
Rounds: 3 6 14 | 2 7 12 | 3 6 12 | 2 6 12