I have been running a tropical fish tank for almost 3 years .
Just wondering if my filter stopped working how quickly would a replacement filter have to run before it was running properly.
Would not want any harm to come to my fish.

If you replace it with the same make of filter you should simply be able to exchange the filter pads in it for the old ones in the filter that broke with very little chance of harming the fish.

even if it isnt the same type of filter, the sponge media can easily be cut down to a small enough size to fit into any other filter between the medias, or if you wedge it in behind the new filter in the tank the bacteria will soon colonise the new media. theres always a way. if you want to replace the old filter with a new one, and the old one is still running, run them side by side for a couple of weeks.

I've read the OP in a different way, maybe : how long do the bacterial colonies take to recover after they've been denied circulating oxygenated water for X hours or days?

I found it impossible to answer directly without having some of the variables tied down through knowledge of WJ's particular tank.

I (and I'm sure you, too) could make some general observations ... but probably not in a sentence or two!

I have 3 internal filters working in my 4 foot tank, just in case one fails, my Lake Malawi cichlids are just to valuable to take chances with.

i run 3 large internals and 1 external fx5 on my 6ft tank, and 1 internal and an external fx5 on my 4ft tank. my goldfish especially need big filtration, and i know if something happens to 1 filter chances are for a few days the other 3 filters will cope. i dont run tanks with less than 2 filters...just incase

Thanks for the advice Kelly82,going get a replacement internal filter 2moro.
Hate to see anything harm the little Fishes.