Until or for to indicate time period starting now

Which of these options is correct?

1) I shall not be home for two hours from now on.

2) I shall not be home until two hours from now on.

What's the main difference between the 2 sentences, using for or until ?

Your use of "from now on" implies that there will be multiple similar cases in the future where you also won't be home for two hours. Moving the clauses around might make it a little more obvious:

From now on, I shall not be home for two hours.

However, when you use until, you couldn't possibly be talking about multiple future scenarios since you're referencing a very specific time in the future. If you drop the "on," however, things begin to make more sense.

I shall not be home until two hours from now.

Sounds good to my native English ear. However,

I shall not be home for two hours from now.

Doesn't sound as natural.

TL;DR

If you're talking about one specific instance, drop the "from now on" and just use "from now" with until. While if you meant to talk about many future situations, use "for" with "from now on."