waiting for sth/sb
eg.I'm waiting for you.我在等你.
I am waiting for your reply.我在等你的回音
for后面跟的是一个很具体的名词,可以是东西也可以是人.如果是时间是不加for的；
eg.wait a minute而不是wait for a minute；
wait a few minutes.
这句话的意思是你经常让我等很久.
楼上的那句话：You often keep waiting for me a long time.感觉语义不太通,个人认为如果一定要加for：You often keep waiting for me for a long time.
just for reference.