Quote Originally Posted by DDT
I thought about "давно" here too but decided that it would mean that I read the words not long ago instead of what I intended, which was, I have been reading your words for a long time.
Oh, I see what you mean. It's an interesting question. Here's what I think:

Я давно читал твоё(и) сообщение(я) = I read your message(s) a ling time ago.
Я давно читаю твои сообщения = I've been reading your messages for a long time.

Я долго читал твоё сообщение = I spent a long time reading your message (because the message was big or it was pretty difficult to understand or I read it slowly for some reason etc)

Я долго читал твои сообщения - sounds ambiguous to me, it could mean
*1) I've read your messages for a long time
2) It took me a long time to read your messages

Я долго читаю твои сообщения - sounds ambiguous as well
*1) I've been reading your messages for a long time
2) It takes me a long time to read your messages (because they are usually long or make me think or pretty difficult to understand etc)

Я долгое время читал твои сообщения = I've read your messages for a long time.
Я долгое время читаю твои сообщения = I've been reading your messages for a long time.

* - sounds not good to me.