Please explain the proper method to perform Pranayama during Sandhyavandanam. How long should one inhale the breath, exhale it, and how should one recite the Pranayama mantra while holding the breath? 

When performing Pranayama, the mantra must be recited as Upamsu, meaning it should be done meditatively within the mind. If spoken aloud, the air escapes. Pranayama (Prana + Yamana) effectively means controlling or holding the breath. One should perform it only for as long as they are able to hold their breath. One should hold the breath, recite the mantra, and then exhale. In the initial stages, when teaching young children, it must be spoken aloud. Once the mantra is memorized, one should hold the breath and recite it mentally.