Namaste everyone!

I've been meditating daily for 8 months. I meditate at night before sleep for roughly 30 mins and in the morning for 5 mins after Surya Namaskara. I feel now as if I need to lengthen my nightly meditation. I've been able to sit for longer and maintain concentration but at the same time I often feel an antsy urge to get up even when I feel myself getting into a deeper meditative state. Is there an ideal length for meditation? A most effective length that enables one to get deeper into a meditative state? I've been doing Jaggi Vasudev's method of mentally saying "I am not the body" on the inhale and "I am not even the mind" on the exhale, then mentally or vocally chanting Om and the names of various goddesses and gods, then Ramana Maharshi's "Who am I?" method at the end. Any suggestions or advice? Thank you.