The time it takes for glacier to form depends on certain factors. The most important of them is the temperature around the location. The rate at which snow builds up is also another factor. The more snow falls in an area the faster it will be for glaciers to form in that area. Glacier is likely to form faster in Alaska or Canada compared to a place in Africa.
Due to the variable changes in temperature in different part of the earth and at different times of a year, it is difficult to determine how much time it will take for glaciers to be formed.
On the average, it takes about 100 years before glacier begins to form.