For morning session practise practise, always sing a note (Sa) for a long time. Go one note down and sing on a sustained way. Do like this in the lower octave till whichever note you can reach in the lower octave. This warms up the voice and also gives power to the voice over a period of time of practice.
Then try singing sarali , jantai , alankaram in akaram. Advanced students take up one melakarta raga a day and trying singing the above mentioned varisais.
Then practice varnam in first speed, tisram and second speed. All the above mentioned practise should not take more than 45 mins.
Evening sessions, when the voice generally has a good reach, you can practice whatever you have learnt. For advanced students, you can take up ragam / neraval / pallavi singing / swaram. Like this one can improve his/her manodharmam.