How long would it take to crack a double hash if salt and final hash are public? I have a hash of a file $H_1$ and then I add salt and rehash it: $H_2$. I make my $H_2$ and salt public and I use my $H_1$ as a password. How long would it take for this to be cracked with someone trying to figure out all the combinations of the 128 characters and adding the salt to get $H_2$?

So they are trying to figure out $H_1$ and adding salt to confirm that it matches $H_2$. As $Hash(H_1 \| salt) = H_2$, how long would it take to crack the 128 characters to find the $H_1$ that corresponds with this combination?

Please assume SHA3-512 as hash function.
A SHA3-512 hash outputs 512 bits. If you use that as input to the second application of the hash then it takes $2^{511}$ tries - on average - to brute force it. This is obviously far out of range of a practical attack.

The one thing that could be performed is to try to guess the input of the first hash: the file content. If this can be guessed then the scheme would be broken. The file could be very short or it may just not contain enough entropy, regardless of the size. For instance, if you'd use the copy of the MP4 version of Kill Bill 1 and somebody knows that you like the movie, the hash may be relatively easy to guess.