She could stay at a similar but slowly worsening level for years and years, it depends on what physical cause underlies the dementia. MIL got very slowly worse after her diagnosis of vascular dementia. It took about 5 years before family members needed to give her a lot of support and another year after that before she was regularly doing things that were dangerous.
Other people find that the decline is much more rapid. Symptoms and problems also vary. Some people have incontinence early on, or hallucinations. MIL got by on "it's just my age you know, I've got a bad memory" for a long time. Her abilities to reason and to be oriented to time, day and place have now gone, but were still with her for several years.
She can still do crosswords, with a bit of help, but she cannot write a sentence any more. Her speech is frequently garbled and a mish mash of bits of sentences and thoughts all stitched together with the right intonation. Then you'll have a couple of hours where she is lucid and able to crack a very sharp joke at someone's expense. My advice is to read up on things but to use other people's experiences as a very general guide, taking from them what is useful to you at any one time. There is a well known saying
"If you've seen one person with dementia, you've seen one person with dementia."