General rule of thumb is that you most likely will spend 40% or less of the cost of the warranty on repairs over the covered period. That is based on the fact that the warranty costs are derived from actuarial analysis of repair costs plus costs added for sales profit and administration.
Also beware that what is advertised as a 5 year plan actually only applies to 2 years if you have a 3 year/36,000 new car warranty.
Larry