Generally installing OS X takes about 20 to 40 minutes, but there are times when the install process may take longer or seem to hang at a specific step. This may be especially true if you're using Apple's new Internet-based installation for Lion. When this happens, even though the installer progress bar has an estimated time remaining underneath it, the time will not change and the bar will not advance.
This mainly happens when the installer is waiting for data to load. The installer will wait for the media to be available, and instead of updating its status it may just sit at the current one and not advance.
Luckily, even though the OS X installer just shows a basic interface, you can get more information about the installation process by viewing the install log, which can be accessed from the Window menu in the installer. With the log open, you can see the exact steps the installer is taking, and can have a better idea of when the process will be complete. Unfortunately, even the logs do not show details like the percentage of files downloaded from Apple's servers, but it does tell you more about what step the installer is at.
Since the main reason for slow OS X installs is the use of relatively slower installation media, if you are planning on installing OS X multiple times then you might benefit from using faster media. For Lion users, download OS X from the Mac App Store and then burn the installation disk image contained within the installer to a DVD (see the). Alternatively and preferably, you can restore either the Lion installer or the installation DVD of an older OS X version to an external hard drive, and then use that to install the OS far faster than any DVD or Internet connection will allow.
To restore the installer to an external disk drive, you will need the installation DVD or a disk image of it, and an external volume large enough to accommodate the data. This volume can be a partition either on a flash drive or on a conventional USB or FireWire hard drive. Format the volume using Disk Utility, and select it and click the Restore tab in Disk Utility. At this point you should see a Source field and a Destination field. For the Source field, drag the OS X installer DVD from the device list to the field, or click the Image button and select your installer image (which is the case for OS X Lion). Then drag the newly formatted destination volume to the Destination field, and click the Restore button.
Keep in mind that even though you can drag the entire drive device to the Destination field, doing so may result in an error, especially when restoring to a flash drive. Therefore, be sure to drag only the desired volume to the Destination field.
When the installer has been restored to the drive, make sure it is attached and then restart the system with the Option key held down. Doing this will bring up the boot menu that lists all available boot volumes, from which you can select the installer and boot directly to it. The installation should now proceed much more quickly than it would from a DVD or the Internet.