When I bought my new computer it used to start very fast, but after some time it started to take longer to boot. I could not figure out exactly how long it takes to boot. Windows does not have any built-in tool to show the amount of time it takes to boot. But you can use BootRacer to find out the boot time. BootRacer is a Windows application, which shows you how many seconds it takes you to boot in Windows.
After installing BootRacer, you can just click on the Start button in the BootRacer window. It will ask you to restart your computer, so that it can calculate the boot time. Choose yes to restart your computer. From the next boot, it would start calculating your Windows boot time. Under the Advanced section, you can configure it to calculate boot time every time Windows starts or just when you want (by clicking on the Start button).
BootRacer calculates the boot time from the Windows startup. First point of detection is the Windows logon or welcome screen. BootRacer notices when a user enters credentials and logs on to user account. After this the Windows desktop and Windows Start Menu are shown. Then the BootRacer determines the boot time and shows it in the bottom-right corner of your desktop screen.
BootRacer also keeps a log of historical boot times for your Windows computer. You will find the History button in the main window of BootRacer. If you want, then you can click on this History button and check all the boot times for past few boots. The time taken is broken down into different sections like time to logon, time from logon to Windows desktop etc.
So if you are curious how long it takes for your computer to boot into Windows, then you can check BootRacer out. You can download BootRacer from http://greatis.com/bootracer/.