Hey I just found a VERY LARGE plastic tank like container, I was wondering whether It is safe or unsafe to keep the fish in such a thing. It is ugly I know but much bigger than a tiny bowl

How long can I keep them in there for?

TBH they look VERY nice in there, plus I have a sponge filter

Why so much?

50% weekly is required, because goldfish are messy little things
They are basically poop machines and due to this, a lot of ammonia is created, in a cycled tank the ammonia is converted to nitrites and then to nitrates by bacteria living in the filter and some areas of the tank, so water changes are required to lower the nitrate level as this can be harmful in high concentrations of 40ppm or more.

In an un-cycled tank, when the ammonia isn't being converted because the bacteria have not yet developed, more water changes are required as ammonia if left, is lethal and can kill fish. For example, I am currently changing my water 3 times a week (I do two 30% changes during the week and one 50% change at the end of the week) this keeps the ammonia under control and my goldfish healthy

On the subject of plastic tubs, I have heard of people keeping fish and turtles in rubber maid containers, so I am sure it is possible.
The length of time they can stay there, really depends on the size of the tub and how fast the fish grow