You might have installed Windows 7 from scatch. Since then, you notice that Windows Update will take hours or even days while checking for updates, It can also occcur if you have not checked for updates for an extensive period of time or you installed Windows 7 system long ago without checking for updates. Even you have you may have Windows 7 SP1 installed.
According to Microsoft website, this issue occurs because Windows Update itself nees two updates, creating a bit of a catch-22. As soon as you these two updates are installed. all other updates should work without any problem.
Here is the instruction from Microsoft about how to fix the Windows Update Stuck
1. Go to Windows Update page
2. Change Setting to "Never Check for Updates (Not recommended)"
3. Restart computer
4. After the computer restarts, download the following update manually and install them one by one.
5. Install KB3020369 first
6. Install KB3172605 second, then restarts the computer
7. After the computer starts, wait for 10-15 minutes before going to next step
8. Go back to Windows Update page
9. Change Setting back to "Automatic"
10. Check Windows Update, it may take about 15 to 30 minutes to find the update.
For 64-bit editions of Windows 7, download these updates:
KB3020369, April 2015 servicing stack update for Windows 7 (64-bit version)
KB3172605, July 2016 update rollup for Windows 7 SP1 (64-bit version)
For 32-bit editions of Windows 7:, download these updates:
KB3020369, April 2015 servicing stack update for Windows 7 (32-bit version)
KB3172605, July 2016 update rollup for Windows 7 SP1 (32-bit version)