同意你的看法，这是一个省略句：

How long before we can see the new ski lifts in action?

完整说法应该是：

How long is it before we can see the new ski lifts in action?

句子大意是：我们要等多久才能看见新的滑雪电缆车投入运行？

这类省略没有什么规律，视具体情况而定。这类句子最“省略”的说法是：

How long?

根据情况可以多种补充：

—I got to know Mary in Paris.

—How long before you got married? (=How long was it before you got married?)

—The boss says that we can go to London for a holiday.

—How long before your holiday starts? (=How long is it before your holiday starts?)