You should always train at a level that requires 100% concentration to maintain good form. If that is all drill right now, then it is all drill. If that is full stroke now, then it is. For most people, each set will contain some drill to set the proper sensation and some swim to maintain it.
And, the more frequently you can train, the better for technical improvement.
The details depend a lot on where you are now.
I like sets like
75 drill - 25 swim
50 drill - 50 swim
25 drill - 75 swim
In a set like this, hold one focal point for the whole set. Let's pretend your first focal point is to feel the hips break the surface at all times. If you notice your hips fall at 75 yds, then do a set of 75s and 100s until you can hold that focal point for 100 yds. Then do a set like
50 - 100 - 150
As soon as you feel your hips fall, do a 25 drill to get them back up and finish your set with the improved focus.
In short, you are training your brain to be able to focus more than you are training your body to work for that distance.