★ When...? and How long...?

Compare:
→When ...? (+ past simple) and 
→How long ...? (+ present perfect) :

☆Q: When did it start raining?
★A: It started raining an hour ago / at 1 o'clock 

☆Q: How long has it been raining?
★A: It's been raining for an hour / since 1 o'clock        
            
☆Q: When did Joe and Carol first meet?
★A: They first met { a long time ago. / when they were at school. 

☆Q: How long have they known each other?
★A: They've known each other ｛ for a long time / since they were at school.

☆Q: How long has Kate been learning Japanese?
★A: She has been learning Japanese for two years.

☆Q: When did Kate start learning Japanese?
★A: She started learning Japanese in 2015. 

☆Q: How long have you known Simon?
★A: I have known him since we were kids.

☆Q: When did you and Simon first meet?
★A: I met him when we were in kindergarten. 

☆Q: How long have Rebecca and David been married?
★A: They've been married for a long time.

☆Q: When did Rebecca and David marry?
★A: They married a long time ago.