I experimented a little with my mage to see if the information on TibiaWiki is correct.
According to TibiaWiki the amount of ticks that a soulfire rune should do is your level + magic level / 3
In my case this should be 464 + 118 (582) / 3 = 194 ticks from the soulfire rune, every tick is 10 damage, so that should be 1940 damage according to Wiki, however this was the result from the first test I did:
As you can see, I did only a total of 62 ticks (620 Damage) from the soulfire rune, not 1940.
I did another test, with +6 extra magic levels:
With 6 extra magic levels I managed to get in another 10 ticks, (+100 damage) for a total damage of 720.
I know that this will not answer your exact question of what the cap for a soulfire rune is, but this should show that the information that Wiki provided is not entirelly true. Magic level does indeed boost the amount of damage you deal by alot, but I could only reach 11 minutes with my second test, perhaps someone with much higher magic level could try reach 15 minutes and see if it stops there or not.