Dissolving 700 grams of Ammonium Sulfate in 1L of water for NAP Buffer can be challenging, and your experience aligns with what Camacho-Sanchez et al., 2013 describes - it can take hours at low to moderate heat. Here are some suggestions that might help:

• Double-check Temperature: While 55°C isn't excessively high, ensure your hotplate stirrer temperature is accurate. You can use a separate thermometer to verify the temperature of the solution.
• Adjust Mixing: Try a more vigorous stirring rate. This can help improve contact between the ammonium sulfate and the water, accelerating dissolution.
• Try Gradual Addition: Instead of adding all 700 grams at once, consider adding it in smaller portions. This can help reduce saturation and make the dissolving process more efficient.
• Extended Heating: If the above suggestions don't work, you might need to extend the heating time. However, be cautious of exceeding moderate heat as it can lead to decomposition of some buffer components.