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Personally I'd give 30GB to Windows, but it depends of you are planning on installing any applications.
 
It depends on what you want to run in Win 7. The installation itself takes up about 20GB. I'd give it at least 32GB plus whatever space you need for applications.

Note that Boot Camp uses the old base 1024 gigabyte instead of base 1000. Therefore, your 128GB SSD, which is really about 125GB because of overprovisioning, winds up being closer to 116GB in Boot Camp Assistant.
 
lol thanks for the replies, looks like I will have to wait for 256gb to come down in price. Work related applications I need to run in Windows alone is 15gb+
 
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