___does the bus come?(How often\How long) Every 20 minutes.

___does the bus come?(How often\How long) Every 20 minutes.
___does the bus come?(How often\How long) Every 20 minutes.

___does the bus come?(How often\How long) Every 20 minutes.
how often
根据后面的回答可知是询问频率的,因此用how often
how long 的回答应该是for短语类的

how often