If you choose a cloud or hosted deployment, the installation will be done for you. The only thing you will need to do that resembles installation is to install your customizations and set up your users.
For on premise installation, the cost can vary significantly. We have spent from one day to two weeks or more, depending on the number of environments to be installed and the state of network and SQL Server infrastructure. One of the factors that causes delays is when many people are involved in managing different servers and networks. In a large enterprise, you can end up bouncing among people who have various permissions, trying to find the right person to do the right installation task.
If you are in a virtualized environment, you can save time by using standard server images and copying them for multiple Dynamics CRM environments.
We have employed a technique which radically decreases the time for installation. You assemble all the people who have access permissions and knowledge of your networks and servers which are involved in the Dynamics CRM implementation in a conference room, along with food and drinks to sustain them through the installation. Next, you perform the installation, knowing that you can call on the right person who has the power to finish a task. This approach cut the installation down from days to a couple of hours for us.