I don't know that there's any exact measurements out there, but "long pattern" wrenches are typically a couple inches longer than their "conventional" counterparts.
As an example:
A Snap-on OEXM150B 15mm "standard" length wrench is 231mm (9.1") long.
A Snap-on OEXLM15B 15mm "long" length wrench is 266mm long (10.5" long.
So, only 1.4" difference there, but, eh?
My Craftsman Professional wrenches are noticeably longer than my Craftsman raised-panel wrenches.
Personally, I prefer the longer wrenches for general work, and only use the shorter stuff if obstructions are an issue.