Kerry I agree with you totally - 30 minutes is [too long!] the maximum "allowed" period. It's what they write in the caudata.org article about salt treatments for removing fungus from skin ("skin mycosis" in the article at
http://www.caudata.org/cc/articles/salt.shtml ).
I also would risk saying again that 1 teaspoon salt per liter is aggressive enough for treating a fungus. I believe that stronger solutions not only hurt (as in cause pain) the axolotl, they can damage it, too (e.g., shedding/bleeding from the gills).
I would suggest that it is better to start with a weaker solution for a shorter time and if this is not apparently effective after a bit (two days, three treatments - just a guess, does anyone have concrete advice here??), the dosage and/or duration can be increased. However, I have neither scientific knowledge nor practical experience to justify this theory - it's only an opinion.
I also believe that if an axolotl is given consecutive salt bath treatments and the condition does not improve or worsens, it would be wise to find a vet with amphibian experience, as the issue may be bacterial and not "just" fungal.
Furthermore, since I have the keyboard here, I will mention that there are also the usual culprits to suspect: water quality, food sources, infection from newly introduced tank mates, etc. The somewhat accusatory "poor husbandry" is often at the root of all evil...