How long can a black ghost knife fish go without being fed?

Oh yeah, he'll be fine for three days no problem. Whether your other fish will survive HIM for three days, however ... they tend to make a go for their tank mates (if they're small enough) when they haven't been fed for a while. If you have any fish in there that are anywhere near bite-sized (that he hasn't eaten already), I'd put them in a separate tank until you get back.