Three sandhya vandanams to be performed everyday.
Pratha sandhya vandham It should be performed during sun rise. If you do after this prayaschita argyam to be done [usually it can be done from one hour of sunrise].Example if sun rise is 6:15 am then the prayaschita should be completed within 7:15 am.
Madyaniham--It should be done by afternoon 12 pm. But nowadays ìn office we cannot do. Hence Kanchi Paramaacharya said that it can be done by 8/45 am.
Sayam Sandhya vandanam--At sunset it has to be done. After returning from office till 9pm you can perform this by doing prayaschitham argyam. Nowadays due to traffic jam and all you can do till 9/30 pm the prayaschitham as elders say that it should not be missed.