It really depends on what the believed cause of the dementia is the following website should be helpful but to give a time frame when we are talking about memory as far as I know is different for everyone. If it is being caused by strokes and the stokes keep happening then it may progress at a faster rate.
Like the PP said, it depends on the cause. There is an early onset Alzheimer's which is much less common than the late onset. (I think early accounts for 10% of Alzheimer's cases?)
I've had two family friends (both male) who had early onset Alzheimer's. From normal functioning (still working and participating in family life) to needing nursing home care was about 4-6 years, but this is variable.
I have an aunt with early onset dementia, of unknown cause. This is anecdata of course, but she's been dx for about 5 years now and is still functioning and raising a teenage daughter. I believe there are medications she takes that are supposed to slow the progression.