Hi Willcooper,
It is impossible to say 'how long will it last' that would depend upon the usage of the plants in your tank.
As for the 'availability' of iron you are correct that PH and the type of chelation effects the amount of iron available to plants. The Plantex Nutritrace (Previously CSM) mix uses Sodium ferric ethylenediaminetetraacetate (EDTA) as the source for its Iron and it makes up 7% of the mix. EDTA chelated iron is better than citrates of iron, or HEDTA chelated iron with regards to availability however at a PH of 7.6 it is likely the amount of available iron in the mix is reduced by over 50%.
Plantex Nutritrace (CSM) micronutrient mix (where's the B (Boron)? Hopefully added by the secondary supplier such as aquariumfertilizer.com, greenleaf aquarium, Bob's Tropical, or Nicolog (a user on this site).