For how long is research considered valid for publication?
Research does not have an "expiry date", but what it does have is competition. Journals want to produce cutting edge research, new information that will help others in the field.

If you do some research but don't publish it, then find 20 years later that no one else has done it and your information is still something other researchers would want to know then you can easily publish it. Delaying publication is only a risk in that other people may work on the same topic or have the same idea you had, and publish it before you.

One interesting example is Brian May, guitarist in the band Queen. He was studying for a PhD when the band took off and quit his studies in 1974 to become internationally famous. In 2007 he re-enrolled at university, completed and submitted his PhD on the work he started 30 years before. Luckily for him, his field was a small one and no one else had decided to work on the topic he had studied in the intervening years, so his work was still new, interesting and relevant.