Some lab technicians have been suggested 2 weeks storage at refrigerator and 6 month in freezer -80 .but in my opinion lyophilized powder of plant extract can be  keep at -20 for more longer times. 

The plant extract samples can be preserved at -20 for few days to months. It is advisable to isolate and characterized the compounds as early as possible.

In case of aqueous extract, you have to take special precautions as it is prone to fungal contamination. If the solvent is properly removed from the extracts and no traces left, then storing in a freezer will keep them intact for quite a few months. Only problem which may interfere in the isolation and characterization is the formation of artifacts of natural products. You cannot predict which compound will behave in what manner upon standing (even on freezing). Terpenoids are already oxygenated derivatives so it would be better to isolate them at the earliest possible and characterize them. Regards 

Crude extracts tend to degrade early - try storing lyophilised extract or plant parts freeze dried in liquid nitrogen in air tight containers at -20C - that would prolong the time.

Aloha...

Go for -20ºC and conserve it dried from any solvent, you should not have problems for several months.

I never had problems and differences with -20 storage, but it depends what kind of extract you have. I think the best is the lyophilised. I kept evaporated methanol and chloroform Soxhlet extracts of Asteraceae plants in  -20 for a year the most. There were no significant differences in the antioxidant activity, the total phenols and flavonoids, and the terpenoid fraction compared to the lyophilised plant analysis, kept in a refrigerator at -4 (for 6 months the most).

I do believe that lyophilised extract can be kept in a simple fridge, at -4 for a longer period, provided you sealed it neatly.

Dear Zohreh Sohrabi

You may be interested in following the threads:

Good luck.

https://www.researchgate.net/post/after_drying_the_sample_how_long_can_it_be_stored_before_will_be_extracted

https://www.researchgate.net/post/Which_do_you_recommend_fresh_or_semi-dry_or_dry_biomass_tissue_for_an_aqueous_solvent_extract_or_for_extracting_isolating_separating_phytochemicals

Dear all, what about storing the dried extract which dissolved in a solvent? I want to prepare master stock solution of plant extract in DMSO. How long dried extract dissolved in DMSO can be stored in -20 or -80? Does it need to be 100% DMSO?