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 entirely true. Magic level does indeed boost the amount of damage you deal by a lot, 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.