The limit is set by the water, not the appetite
Ask how long a goldfish can be left and the limiting factor is rarely the fish's stomach. A healthy goldfish tolerates a few missed meals with ease. What it cannot tolerate is its own waste accumulating in water nobody is watching, because this species produces more of it than almost any comparable pet fish, and an ageing filter or an overfed day can tip a stable tank within the span of a short trip.
With a real once-daily check, several days away is reasonable for a healthy goldfish in a generously sized, well-filtered tank, and around a week is a sensible outer limit for a novice checker. Beyond that, arrange for the checker to be able to test the water and carry out a partial change you have rehearsed with them, or upgrade the arrangement to a knowledgeable sitter. The calendar matters less than the competence attached to it.
What the daily check must actually include
Brief the checker on a short, physical list. Watch the goldfish swim and confirm it is level, active, and interested. Confirm the filter is running and its flow looks normal, because a silently stalled filter is the classic mid-holiday goldfish disaster. Read the temperature where a heater is fitted. Feed the measured portion, once, and remove any food still uneaten after a few minutes rather than leaving it to rot into the water the fish must breathe.
That last duty deserves emphasis in the brief. Goldfish beg convincingly at any face, and checkers who feel sorry for a hungry-looking fish undo more holidays than equipment ever does. Portion the food in advance, day by day, and write plainly that the begging is theatre and the pot is the law.
Set the tank up to forgive a week
Do a partial water change and a filter check a couple of days before leaving, so any disturbance settles while you are still home to see it. Gravel-clean the worst of the waste traps, confirm spare batteries for any air pump are in the drawer, and leave the test kit out with the one reading that matters most marked on the brief, along with the instruction to call you if it strays rather than attempt corrections alone.
Consider the fish's company, too. A tank carrying tankmates at its comfortable limit has less margin than the same goldfish alone in space, and holiday feeding by an enthusiastic checker consumes margin fastest of all. If the tank runs close to its limit in ordinary weeks, shorten the trip or strengthen the cover; the fish cannot tell you which week its water chose to turn.
Fish that should not be left on checks alone
A goldfish with any buoyancy wobble, a recent appetite change, healing damage, or great age should not spend even a short trip on once-daily checks. Its problems develop at night and on the second day, not at visiting hours, and it needs a sitter who is present, or a stay where experienced eyes are. The same applies to a small or newly established tank, where the water has neither the volume nor the maturity to buffer a bad day.
Come home to a test, a look at the fish's trim and outline, and a partial change before normal feeding resumes, and file the checker's report with the trip plan.
The whole arrangement stands or falls on the checker following it exactly, which is why KinsLe delivers it through its Care Pass, handing them temporary, clear access to the goldfish's portions, filter checks, and call-you thresholds while you travel.