Q:

How long should one hold the mudras during the recitation of Gayatri mantra?

A:

It is not about the time duration. There are two vibrations that constitute the universe, one of sound and the other of light. Mudras represent the light and Mantras the sound. Together they create a complete experience of divinity.

Gayatri Mantra constitutes 24 Syllables

Om

tat savitur varenyam (8 syllables)

bhargo devasya dhimahi (8 syllables)

dhiyo yo nah prachodayat (8 syllables)

(Rig-veda 3.62.10)

Each syllable is a sound form, has a related light form, this light form is the Mudra. You can find these 24 + 8 Mudras along with other explanation in my book, Mudra: The Sacred Secret

There are 3 ways to do the practice:

1. Each Syllable as meditation: When practicing Gayatri Mantra the practitioner must mediate upon each syllable as if mentally writing it, inscribing in on the heart and practice the Mudra associated with each syllable. Each Mudra together with the syllable utterance (mental or audible) invokes a specific state of mind and eventually brings about a sense of transformation. Pay attention to the sound of the syllable and hold the mudra until you feel the sentiment of the sound in your heart. Take your time. Do not rush to complete the Mantra. This type of practice slowly leads to the realization of the Mantra.
2. With Recitation: Recite the mantra and practice the 24 Mudras of Gayatri along with it at the pace of recitation. It takes time but with regular practice you will be able to synchronize the hand movements along with the Mantra. After completing the Mantra, practice the other 8 Mudras of Gayatri Mantra. You may choose to repeat it as many times as you feel like, however if it is done once with sentiment, it is enough.
3. Before the Japa: Prior to the Japa of the Gayatri Mantra, meditate upon Gayatri as Goddess of light, wisdom and life itself. Practice the Mudras as if you are requesting the Goddess to take a seat in your finger tips. With this intention practice the Mudras and then set the intention for the mantric energy to be established and seated in your heart space. Now, continue the Japa and practice as many multiples of 108 as you are inspired to.

Practice Tip: Fingers are considered the rays of solar energy. The rays of solar energy are considered the fingers of the sun. During the Mudra practice let the rays emanated from your finger tips merge into the solar energy and the solar rays enter your finger tips. May the entire body become the body of Prana. When we practice Mudra with such knowing then the practice becomes spiritual.

Gayatri Mantra album available here