This is a relevant article on the subject.

https://www.nature.com/articles/nature.2012.11555

>After cell death, enzymes start to break down the bonds between the nucleotides that form the backbone of DNA, and micro-organisms speed the decay. In the long run, however, reactions with water are thought to be responsible for most bond degradation. Groundwater is almost ubiquitous, so DNA in buried bone samples should, in theory, degrade at a set rate.

Basically, unless the sample is preserved under unnaturally dry conditions while also conveniently protected from all other sources of degradation (something that is unlikely to happen naturally), you're looking at best case scenario of ~7 million years, though the most likely would be ~1 million years.

As water is the main contributing factor, it's unlikely salt water would have any significant net positive effect on preservation length of time.

Edit: Here is an article I found that suggests high salt anoxic conditions will have a net protective effect, however the study does have some limitations. Overall, it might suggest that a high salt environment might help skew the half life to the longer end of the spectrum, though I'm unable to suggest it may extend it beyond that to any significant degree.

https://www.nature.com/articles/srep22960

Also I didn't truly answer your question. I would expect that we would be able to extract DNA from the organisms you are looking at in those salt pools, however the success would decrease with time. After about 500 years, about 1/2 the DNA bonds would be broken, and 500 years after that another half would be broken. We can piece together fragments of DNA to make up the whole (sort of like piecing a puzzle back together), but the chance of success would decrease with time. The further back you go, the more samples you'd need to get a complete picture, and after about a million years the task would pretty much become impossible even if you had an abundance of well preserved samples.