How long have you been here?你在这儿多久了?
How long have you been in the army?你在部队服役多久了?
how far 问路程“多 远”；
How far is it from Beijing to Guangzhou?从北京到广州有多远?
how long 是问（时间）“持续多久”,常与现在完成时或一般将来时连用.
综上所述：选B.