It varies a lot and depends on many factors such as ICO itself and its team, technical issues, marketing. The crypto market situation plays a significant role as well. During the bearish market, the number of listed ICOs significantly reduces. Usually, they can be listed as early as 1 month or as late as 6-7 months after the launch. On large and reputable exchanges like CEX (https://cex.io/buy-bitcoins), the process will take a few months, to make sure that a particular ICO meets all the requirements.