Long notes require support, movement, dynamics and a sense of pulse other wise they are just boring to listen to and sing. The sound is held on the vowel and the final consonant sung as and when the MD wants that. Only sing the final consonant until the last minute. Consonants do not carry pitch and are difficult to sing on anyway, consonants are important but must not disrupt the flow of the vowels and line of music.
It’s not just a matter of counting the length of the note to be sung but also thinking of keeping the pitch throughout the note as it does tend to go flat especially if at the end of a phrase when breath is running out. Pre planning for the length of the phrase at the beginning of the phrase will help sort out how much breath you will need.
In order to keep the sound on pitch and strong, keep your chest (not your shoulders ) lifted and think of a strong column of sound, keep the space in your mouth as allowing that to become smaller as you progress through the long note will make it go flat.
You must watch the MD and stop singing exactly when he wants you to otherwise it will sound odd if some stop singing before the others. If you find it hard to look up from your music you must make sure you count the length of the note as the MD will stop the sound around that counting. Ideally as you prepare for the phrase, and after it has been sung a few times in rehearsal, you should be able to quickly memorise the phrase and then look up.
Listen to the piano accompaniment for the pulse throughout the long notes, that will help create some movement to keep your tone engaged. Remind yourself what you are singing about and most long notes will have a small crescendo and decrescendo. In order to keep the pitch of the note you can think of holding the note up on plate in front of you and not allowing it to drop. Or another image or movement will work for you if you come up with your own. When placing the final count of the long note think of giving it a little accent with the final consonant.
Exercise
Aim to remember each phrase before you sing it so you can look up and watch the MD. Be mindful of what it is you are doing to keep the pitch from dropping.