Answer: The tradition prescribes the Sandhya Vandana to be performed thrice a day namely, morning, evening & noon. The noon ritual is known as Maadhyanikha. Most of the people who are working or studying in schools/colleges will not be able to perform in the noon. They may do only morning and evening. If for some reason, it is not possible to perform morning and evening, some do only once i.e. in the morning after taking bath. If even this is not possible, a person can perform the japa of Gayatri Mantra as much as possible. The Gayatri Mantra is the core of the whole of Sandhya Vandana while other aspects relate to more of rituals. It would be more beneficial if one does it knowing its meaning. In case you do not know the meaning well, there are books available in our Ashrama and else where on the subject. Sri Ramakrishna says that Sandhya merges in Gayatri and Gayatri merges in OM.
The main purpose of Sandhya Vandana etc is to think of the Lord at least during some part of the day. It helps us to develop our concentration, will power and character.