What should be bore in mind when playing long note exercises? Should I play until my breath is exhausted? ... I can't control my embouchure whenever my breath is almost exhausted...

I seem to be posting a lot recently... forgive me if my posts bother you.

Learn what a full breath is , pulling out with your abdominal muscles to expand the lower part of the lungs the diaphragm will pull down, open rib cage( keep elbows away from body) to fill middle part of the lungs then fil to full capacity ( not always nessasary when playing though ) Push in with the abdominal muscle, the diaphragm will support the pressure in the lungs. Keep pushing with the abs. Watch how someone breaths naturally when sleeping on there back the stomach rises and falls as you breath. Stomach pooches out as you breath in and sinks in as you breath out. With that comes breath control and support of ther sound. Proper practice makes perfect.

When I play long note exercises, I set a meranome for 60 and hold the note for 12-16 counts. Then I take four counts to exhale and inhale. I don't think it would be a good idea to hold the note for too long

I concure with oboegirl , take frequint exhale and inhale breaths Still learn to take a full 3 stage breath just to learn how the apperatus works.It happens in one motion. like 3/4 time in 1.

I recommend taking a few deep breaths without your oboe concentrating on correct breathing techniques and then pick up your oboe and play some exercises standing some sitting and even play in front of a mirror. Make sure you have good posture and this will help with those long notes.

One of the most important things in long tones is to listen to your tone quality. Listen and hear.