Unfortunately, no one has a crystal ball so you can never be sure how long you could need long-term care. We can look at some statistics to see what some of the nursing home "averages" are, but that is no guarantee that if you needed long-term care you wouldn't be 'above average', or 'below average'. 

Length of StayPercent of Nursing Home Patients
Less than 3 months17.8%
3-6 months10.1%
6-12 months14.9%
1-3 years30.3%
3-5 years13.6%
5+ years14.0%
Source: The National Nursing Home Survey: 1999 Summary National Center for Health Statistics US Department of Health and Human Services, June 2002
There have been a lot of studies on the average length of stay in a nursing home, but it is difficult to get studies that show how long people need care in assisted living facilities or in their homes.

An important point to note is that long-term care usually starts out in the home. Therefore, it is important to consider the length of time you may need care at home BEFORE you entered a nursing home and not base your planning on only nursing home statistics.

The average caregiving time in a national survey was five years.
Health Spending Projections For 2001-2011: The Latest Outlook, Office of the Actuary, Centers for Medicare and Medicaid Services, Health Affairs, March/April 2002