We follow the state records retention laws. Basically everything except the big 'uns gets tossed after ten years. I've had two cases in 16 years where this was inconvenient to the prosecutor as the evidence/prints had been tossed after having reached the limit. One was a robbery, won an appeal waaaay after conviction. They retried him anyway, shockingly the jury still found him guilty. And no, no one ever had to testify as to why there was no evidence. The prosecutor just explained it to the jury. It's reasonable.
The other was a burglary, in this case no charges because the prints were destroyed and that was the only connecting evidence. It happens. The same way victims and witnesses sometimes move away and fail to notify the DA's office of their new contact information; cases are dismissed or delayed for that all the time.
Incidentally, we follow the ten year rule primarily because "ten years" or "forever" are the two longest retentions. Some cases (like trespass and MVT) have a much shorter statutory period of retention (3 years as I recall), but to sort through all our files every year to pull out just this case and just that case seems ridiculous so we stick to the ten year rule and that satisfies all requirements.
I keep 6 honest serving men
(they taught me all I knew)
Their names are What and Why and When
And How and Where and Who.