I'm no expert, but I know that in some other traditions (my experience is in Buddhism) meditation is encouraged three times per day (15 minutes each), however this is usually in the form of a breathing meditation or contemplative meditation (such as cultivating Loving Kindness).
Many Buddhists gradually increase their meditation time up to an hour in duration each day, with two or even three hours at the weekend (breaking up sitting sessions with a walking meditation or mindful eating for example). I can't comment on the safety of mantra meditation three times per day as I am new to mantra. Buddhists meditating in this way would not normally be undertaking advanced practices advised here (Spinal Breathing, mudras etc)either.