Just unfired raw clay? As long as it is dry, forever pretty much, it is more a pratical problem since it is brittle until you bisquit fire it.

I mean glazed(decorated) unfired clay.

#2: Same deal, it will not go bad, it is just brittle and annoying to deal with.

I see, thanks . you meant it is hard to deal because it is brittle but it doesn't become brittler over time, yes?

sorry if I am asking the same question ,I just want to be sure that I can keep as long as it needed and it won't be get brittler over time.

Yes, it is brittle from day one, once the glaze have dried it is pretty much gonna stay the same.

I don't know how much experience you have with ceramics, but as a more general comment, I wouldn't start glazing before I know where I am gonna fire the work. Ideally you bring unglazed work and do all the glazing at the same place as the kiln. Since the glaze doesn't stay on very well without additives you must otherwise pack the glazed ware very carefully and somehow transport it to where the kiln without messing up the glaze. If this a problem or not really depends on what kind of work you are making and what glazes you are using.

Thanks so much, I am beginner.
And do you know any place in and near Stockholm that I can take to fire?