Thanks Darvin.  435 sec = 7.25 minutes, so interestingly, same as Revelation's Provoke Hysteria. 

I finally broke down and did some testing of my own.  Unfortunately the results don't always jibe, or correspond with other conclusions.  For example, I also tested several levels on a 6000 health HW.  Including a lvl 3 Vulkoras (2 levels of seige, and 1 assault spec) and it only took ~6.5 minutes.  Not sure why the differences? Other than: possibly Assault Specialization not used in your test, or possibly the Vulkoras was attacking other structures in between seige deployments, or possibly antimatter differences.  I turned off auto attack and forced the Cap ships to also attack the planet, in addition to their sieging abilities.  The following are my results with fastest settings:

Lvl 1 Vulkoras (1 seige, and 0 assault spec)  :  9'
Lvl 3 Vulkoras (2 seige, and 1 assault spec)  :  6' 35" - 7' 15"
Lvl 5      "        3                 2                     :  6' 00"
Lvl 6      "        3                 3                     :  5' 35"

Lvl 1 Marza    (1 raze, and 0 incend. shells)  :  12'
Lvl 3 Marza    (2 raze, and 1 incend. shells)  :  8'
Lvl 5    "         3                2                        : 5' 30"

___________calculated estimates below____________________________________

Lvl 1 Revelation                                         : ~20'             (6000/5)
Lvl 6 Revelation (Provoke Hysteria)              : ~4' - 4' 30"    (remaining 20%=1200/5.0)

4 TEC or Adv Siege frigates (in Diplomacy)    : ~8'              (6000/(3.1*4))
4 Vasari Siege frigates        (in Diplomacy)    : ~7' 35"         (6000/(3.3*4))

1 Colony Cap ship (no abilities used)             : ~25'!            (6000/4.0)
1 adjoining Culture center, 1 lane                  : ~25'             (100%/-.07%)

Going back to calculating potential results I noted that a "salvo" of damage apparently occurs every 10 seconds, so these figures can be converted to a DPS.  The following is a summary of base siege damage for all Cap ships (note, all 3 colony Caps do the same 4.0 DPS seige damage):

TEC:
Kol       43    4.3
Sova    36.5  3.65
Akkan  40     4.0
Dunov  43     4.3
Marza   56     5.6   + Raze Planet 160/320/480 damage, 50s cooldown
  frig     31     3.1

Advent:
Radiance 43    4.3
Halcyon   36.5 3.65
Progen    40    4.0
Rapture   43    4.3
Revel.     50    5.0 + lvl 6 Provoke Hysteria -40% dmg, every 180s cooldown, cooldown reduced by 40% w/ Guidance?
  frig       31    3.1

Vasari:
Kortul     45    4.5
Skiran.   35    3.5
'Egg'      40    4.0 + lvl 6 Drain 1350 every 180s
Maraud. 40    4.0 
Vulk.     53    5.3 + Deploy 3 lvls of sieges (20, 26, 32), + Assault Spec. speeds bombing 25, 50, 100%? 
   frig    33    3.3

Siege frigates were updated after the original discussion (1.18 changelog ), so that they now do more damage, cost less in fleet supply (12 now, vs 14), and have more health (HP).  But there are differences between vanilla Sins and Entrenchment or Diplomacy, ie:

TEC Krosov Siege Frigate:

• Planet bombing damage increased from 21 to 25. (Original Sins Only)
• Planet bombing damage increased from 21 to 31. (Entrenchment Only)

Advent Purge Vessel:

• Planet bombing damage increased from 21 to 25. (Original Sins)
• Planet bombing damage increased from 21 to 31. (Entrenchment Only)

Vasari Karrastras Destructor:

• Planet bombing damage increased from 23 to 27. (Original Sins)
• Planet bombing damage increased from 23 to 33. (Entrenchment Only)