long before在用法上完全等同于 long time ago

有一定的区别
Long before 后面要接名词， 代词等等。
如： I had been（或 was, 更适合)there long before you were born...
而long time ago只是短语， ago是副词， 后面不接词。
如： I saw him a while ago(或 long time ago)

long before
在很久以前， 早在……之前， 很早以前
Their marriage was washed up long before they separated.
他们分居前婚姻早已破裂了。
It was not long before tragedy struck again.
没多久，灾难又再次降临。
We had not waited long before she came.
我们没等多久她就来了。
Long before his visit concluded, they conversed with the familiarity of a long-established acquaintance.
在他的访问还远没结束之前,他们已在用老相识的随便态度进行攀谈了。
long time ago
adv. 很久以前， 一段时间后
I forgave her a long time ago.
我早已原谅她了。
A long, long time ago, cowboys were wanderers, driving their herd from place to place, looking for fresh grass and water.
在很久很久以前，牧童都是游牧者，赶着他们的牧群到处寻找青草和水源。
A long time ago the tiny country of Switzerland was ruled by Austria.
很久以前，瑞士这个小国被奥地利统治。

比较下面两句,你的这个问题就解决了：
1 She says,"It happened a long time ago."(直接引语) ----- a long time ago 用于一般过去时
2 She said that it happened a long time before.(间接引语)-----a long time before 用于过去完成时
a long time before 在这里等于long before.,这里time是可数名词,所以前面要写上a------:a long time ago / before.