download time = file size / internet speed.
(assuming a stable internet connection and constant speed (laughs))
File size is normally in GB you need to convert it to bits, a byte is 8 bits, a megabyte is a million bits (don't worry about it not being technically a million but a little over a million you don't need an exact answer) and a gigabyte is a billion bytes so take your file size in GB and multiply by 8 billion.
Now your internet speed will be measured in baud which is bits per second. So just divide your file size in bits by baud in bits per second and you'll get the download time in seconds, you will then want to divide by 60 to get it in minutes divide by 60 again to get hours.
Your computer should do that for you (although more accurately) and it will constantly give you an updated estimate based on the amount of data already downloaded and taking into account (in near real time) the fluctuations in download speed. Your computer should give you that information with the bar that tells you the progress.
Factors that affect your download time include, how many users are using your broadband and how many users are using the same ISP server etc. If you are using wireless other wireless signals could also cause the signal to drop out causing delays as the computer has to re-establish a connection.
So the short answer to your question how long does it take to download leage of legends is "how long is a piece of string?"