Slowly. Most types of dementia progress slowly over a number of years. Early dementia involves problems with memory and a decline in functioning. In moderate dementia a person needs help with multistep tasks. In severe dementia a person needs help dressing, eating, and bathing. The progression is dependent on what type of dementia you have. Rare types can progress much more rapidly.
Depends on type. The most common form of dementia, caused by alzheimers disease is generally slow and indolent in its course. If the dementia is caused by a stroke (or many) the symptoms will be more sudden but may stabilize. Another common cause of dementia, lewy body dementia, is more rapidly progressive then those mentioned and also parkinson's dementia. Frontotemporal dementia presents earlier and moves faster.