This really depends on what type of Leylandii tree you have as there are several different types and it also depends on the quality of the soil, the weather and access to light to which the tree is given access to.

In ideal conditions you will find that Green Leylandii is by far the fastest growing tree if you are looking for vertical growth. This will grow at around a maximum of 3.5ft a year, which makes it the quickest conifer of it's type. However the Green Leylandii variants is a thinner tree than the Gold Leylandii.

The Gold Leylandii will grow wider and bushier, but it trades that by not growing as fast vertically. For Gold Leylandii you will be looking at a maximum vertical growth rate of about 2.5ft per year. The Green Leylandii is by far the best selling tree in the Leylandii type. Most people opt for the faster vertical growth over the wider hedge type Gold tree.

The Goild Leylandii will grow to a maximum height of around 60ft with the Gold stopping about 30ft.