on earth :究竟
how long :多久
顺便说下,应该是 fall in love with sb.这个固定搭配.意思是爱上某人.
本句话的意思：爱上一个人究竟需要多久.若有人这样问你，你认为怎样回答较为妥当？谢谢！Falling in love with sb is a not difficult thing,maybe just need one second,maybe just a glance of sb,your eyes are fixed.But what the difficult thing is how to forget him or her.Hope you accept my answer! thank you !