The states with the greatest percentage or professing Catholics are central states, namely Guanajuato (96.4%), Aguascalientes (95.6%) and Jalisco (95.4%), whereas southeastern states have the least percentage of Catholics, namely Chiapas (63.8%), Tabasco (70.4%) and Campeche (71.3%). The percentage of professing Catholics has been decreasing over the last four decades, from over 98% in 1950 to 87.9% in 2000. Average annual growth of Catholic believers from 1990 to 2000 was 1.7% whereas that of Non-Catholics was 3.7%. Given that average annual population increase over the same time period was 1.8%, the percentage of Catholics with respect to total population is still decreasing.

Based on the above article, answer a question. How many years was the time span where the percentage of professing Catholics was decreasing?
40