Depends on the size of the phone you buy? I would imagine it's close to the same amount available on any other model; the capacity changed from base 10 to base 2 and subtracting the size of the .IPSW file of the OS.
My 32G 4S I ordered came with about 28.5 G free. I'm pretty sure I can handle it - my 3G I used for almost 2 1/2 years still had 4 of 8 GB free at the end.
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So on an 16GB, it would be something like 14GB.