写I haven't seen you for long time .对吗？

I haven't seen you for a long time.

a long time表一段时间
如Three years is a long tine.
long tine表长时间，无具体时间
而完成时只能与一段时间连用
所以I haven't seen you for a long time. 是对的。

其实这都说复杂了，要是想表达“有很长一段时间没看到你了”这个意思，有个最常用最简单的口语说法，就是：Long time no see.

I haven't seen you for a long time 才是对的。
for a long time 是固定搭配

随便一点(口语话一些)的说法：Long time no see

I haven't seen your for a long time 较正式一些的
For a long time 固定搭配

I haven't seen you for a long time 才对啊。