Got this straight off of their website. I have mixed for 24 hours, but not intentionally. Never had any issues with using it after I mix for 8...
Directions
1. Mix salinity™ with dechlorinated tap or purified water. We recommend the use of a commercial dechlorinator like alpha™ to neutralize any tap water chlorine and/or chloramines. If source water quality is poor we recommend purification with a Seachem Pinnacle™ RO/DI system. To prepare small quantities, bring 35 grams of salinity™ up to a volume of 1 liter, or add 36.27 grams to 1 liter of water. [This is a little less than 1/2 cup of salt per US gallon of water. For 15 gallons use 7 cups of salinity™.]
<span style="font-size: 13px">2. Stir well to ensure a good mix. Although the salinity™ solution may be used immediately, we suggest mixing for approximately 24 hours to achieve oxygen/carbon dioxide equilibrium.</span>
3. Measure the salinity. We recommend a salinity of 35‰. This will be a temperature compensated SG (specific gravity) value of 1.026. If you are using uncorrected SG values (H) then the following formula will yield a value for S (salinity) accurate to within 1%:
S = [0.3348 * T] + [1325 * H] - 1330.4525 (calibrated at 15.56 °C (60 °F))
S = [0.2211 * T] + [1325 * H] - 1330.4525 (calibrated at 25 °C (77 °F))
where S is the salinity in parts per thousand; T is temperature in degrees centigrade (range of 13-30 °C); and H is the uncorrected hydrometer reading. If you don't mind an error on the order of ±3%, simply use the following table:
If the water temperature is: cal.@ 15.56 °C cal.@ 25 °C degrees Cdegrees Fthen target H=then target H=13 - 1555 - 591.0271.02916 - 1960 - 661.0261.02820 - 2367 - 731.0251.02724 - 2774 - 811.0241.02628 - 3082 - 861.0231.025
4. Adjust salt level accordingly. If salinity is too low, then add more salinity™. If too high, add more water.
5. Change 20% of aquarium water every two weeks (or as necessary) to maintain optimum water quality.
USAGE NOTE: NEVER mix salt in an aquarium containing livestock. Transfer livestock to the aquarium AFTER salt is completely mixed and specific gravity has been adjusted. Use Seachem's Stability® to avoid "new tank syndrome" and to prevent unnecessary livestock losses in a new system.
CAUTION: SALINITY™ IS NOT FOR HUMAN CONSUMPTION. KEEP OUT OF REACH OF CHILDREN. Contact in dry form may cause skin or eye irritation. In case of eye contact, completely flush eye(s) with cool water and seek medical attention.