Assuming that you have the 15" Late 2011 MBP - MacBook Pro (15-inch, Late 2011) - Technical Specifications, you can go up to 16GB RAM. From the base 4GB to 8GB and then 16GB.
If up plan to switch between OS X and Windows, 4GB may be sufficient, but if you plan to use both simultaneously using a Virtualization solution, then more RAM helps.
From Boot Camp 5.1: Frequently asked questions
Which versions of Windows work with Boot Camp 5.1?
You can use:
- Microsoft Windows 8.0 and 8.1 (64-bit)
- Microsoft Windows 8.0 and 8.1 Pro (64-bit)
- Microsoft Windows 7 Home Premium (64-bit)
- Microsoft Windows 7 Professional (64-bit)
- Microsoft Windows 7 Ultimate (64-bit)
Windows XP, Windows Vista, and Enterprise versions of Windows 7 and Windows 8 are not supported.
Are 32-bit versions of Windows 7 and 8 supported?
No. Boot Camp 5 supports 64-bit versions of Windows 7, 8.0 and 8.1 only. Boot Camp 4 supports 64-bit and 32-bit versions of Windows 7, depending on the Mac model you have. For more information, see Boot Camp: System requirements for Microsoft Windows operating systems.