How far into the future will God know once we're in Heaven?

Well, eternal prescience is impossible. If it were to be possible, which would require infinite consciousness and awareness, he would, like the rest of us, be a slave to his foreknowledge. And if that were the case, then he wouldn't be Omnipotent. He would be a mindless automaton creating what he his knowledge told him to. I for one believe God is the absolute free will. No restraints. He can do anything. His foreknowledge? I believe he found a way to make himself able to see the future.

Since Heaven exists outside of time and space, the term "future" has no meaning.

God does not know what will happen tomorrow.