Hi Rana, quercetin is a natural flavanol commonly found in fruits and vegetables (i.e. onions, apples, berries, buckwheat, citrus fruit) and works as a potent antioxidant in the body.

It has many uses and is often suggested for various circumstances but it does also have some warnings to be aware of:

1. It suppresses histamine release from mast cells, calming the immune system (and therefore allergic and asthmatic responses)*  It’s often formulated with other synergistic ingredients (e.g. bromelain, vitamin C, nettles – better taken on an empty stomach to be a helpful addition).
-Dosage: 500-1000 mg, 1-2x per day as needed (occasionally 3rd dose per day, short-term is fine), starting a full month prior to strong seasonal allergy time.

2. It dilates blood vessels by increasing nitric oxide production and reduces blood pressure in those with hypertension**
-Dosage: 500mg 2 x daily

3. It reduces inflammation from LDL oxidation*** and potentially reduces triglycerides (early data)****

4. Quercetin can help raise low glutathione levels (our body’s most powerful antioxidant and main detoxifier of heavy metals)*****
-Dosage: 500mg 2 x daily

5. Quercetin is contraindicated while one is taking antibiotics. It may inhibit COMT SNPs (part of phase 2 detox), which may increase risk of estrogen-mediated cancers; caution if COMT is already slow.******

Hope this helps answer your question as to when/what it is used for.

References:
* http://www.ncbi.nlm.nih.gov/pubmed/22470478 
** http://www.ncbi.nlm.nih.gov/pubmed/9718001 and http://www.ncbi.nlm.nih.gov/pubmed/17951477 
*** http://www.ncbi.nlm.nih.gov/pubmed/20828867 
**** http://www.ncbi.nlm.nih.gov/pubmed/23359794
******http://www.sciencedirect.com/science/article/pii/S0891584901008127
******https://academic.oup.com/toxsci/article/81/2/316/1656122