How often should I practise singing?

Most days, for a short time, beats one long session a week. Singing is a motor skill, and motor skills respond to how often you repeat something far more than to how long you repeat it in one sitting — twenty minutes daily will move your voice further than two and a half hours on Sunday, and the daily version survives a bad week where the weekly appointment does not. Practising every day is fine for the voice as long as you keep it inside a comfortable range and warm down afterwards.