I am not too sure what you are trying to ask?
Are you asking where the point is that interior decorating ends and where exterior decorating starts?

Just slap it on any old how, the 'professionals' do it that way, which is why those of us who make and fit sash windows are called back to free up something that the paint guys have painted shut.

You're really doing this job at the wrong time of year. You SHOULD be painting whatever you can see and REGULARLY moving the sashes to stop the paint gluing them shut then, when that's dry, you reverse the sashes (top fully down, bottom fully up) and paint what you can see, again moving them to stop them sticking. All this faffing around is why the paint guys can't be assed. In cold weather you haven't really got a chance of the paint drying so you'll have to make do and just go around unsticking the sashes when it has gone off.

Depending on how old the windows are, and if they were made of oak, then the pulley stile (the bit the sashes run against at the sides) was often left unpainted, along with the running faces of the separator bead (the bit in the middle sticking out of the pulley stile).

Once painted and dry run a candle along the insides of the staff beads and the separator beads to lubricate the run of the sashes.

Hammy, no painter & decorator (and my son-in-law's one) is going to turn down a job even at this time of year. Once they've put the paint on the job's done as far as they're concerned and you can hardly expect the customer to leave windows wide open all night. I'm not saying this just to start an argument, but I've SEEN brushies paint the WHOLE of a sash window, closed, and just leave it like that. I had one hell of an argument with one bunch of guys because one was going around sanding down, another came after to prime/undercoat and the top coat was put on in an afternoon (and that was EIGHT windows, four back, four front). There were four of them, two working out and two working in. They argued with the customer that they'd done what they were contracted to do (and they did LOOK great). She paid them. Guess who went back to free them all off? They certainly weren't going to do it. And yes, my son-in-law does do it the right way, as did the brushy who painted the sashes I fitted in October.

To return to my original point... December is not a good time of year to paint sash windows. From the date of the original posting he'll know that by now.