I haven't done much with the game recently aside from firing up a credit now and then when I can't think of what else to play, but today I tried something on a whim and discovered that the bosses have tick points and that they can time out, which is roughly 2:30. Basically, any "core" on a boss after it is destroyed can be milked (only applies to stage 1 and 2 bosses since they have two cores), as can any part of their body that shoots projectiles, regardless if its still shooting or not. From my understanding, the tick points on a boss are:
Stage 1 - Forehead/chest cores, mouth and abdomen turrets
Stage 2 - Eyes/abdomen cores
Stage 3 - Mouth turret (under weak point), Tail(?)
Stage 4 - Mouth turret, turrets on body segments (ones that fire the ripple lasers)
Just as an example, here's after milking the stage 1 boss, and before the 10k from killing it is applied. I had 87.3k before the boss so I squeezed an extra 25k off of him. With more practiced/aggressive milking you could probably get 28-30k off of him alone.
EDIT: Forgot to mention that I tested this on conversion, but dedicated should work the same.