I would say leaving them for around 7 days would be fine if their diet was a good one and they were fed well prior to the last feed but personally, I would always be worried that I took the risk to leave them that long and it would live in my head about losing them. If you are able to, I would definitely get an automatic feeder for your fish and set it to open every few days, so you know they are still getting fed and you do not have that worry hanging over you whilst you are away.