I don't really have a hard and fast ration..
I commit three of my inital seven to military (though early on they help with other jobs), after that I try to get two squads of five to ten fairly early on.
Ultimatley however, once I have the basics covered in my fort (mason, carpenter, lumberjack, cook/brewer/farmer, etc) I put the rest into military. I will initially try to have three fully outfitted squads (one 'bladed', one blunt and one ranged) with the rest as, more or less, cannon fodder with whatever weapons/armor are laying around.. If it goes far enough I sometimes increase to further equipped squads.
But then I am military-centric. I find traps, in their current incarnation (though thankfull it looks like that will change in the forseeable future) to be far too easy. Between cage traps, stone fall traps, weapon traps, not to mention all the non-"trap"-traps you can build such as cave in, water/magma/etc you can fend of nearly any assault with enough traps.. I find it more intersting to use soldiers to try to hold off the incoming hordes (I also play with other limitations, such as no walling/drawbridging myself off, so its even harder).
So I guess in answer to your question, early on I try to start training up my soldiers (don't use danger room exploit, so I try to get them going early), and ultimatley tend to have somewhere around thirty or so trained soldiers and then any extra dwarves who aren't needed for fortress maintenence are drafted into my infantry.
-MB