Under which conditions must one store the solder paste?

Between 0-8 °C; 3-6 months. In closed doors that doesnt hit day light in, you can store solder paste for a short time. Please check the expire date before you use.

Temperature:
Affects the viscosity.
Keep it in a proper cold.

Humidity:
Low humidity may dry the solder paste.
High humidity moustrises the solder paste.
Keep it in moderate humidity...