Sri Chandrasekhara Paramacharya of Kanchi Kamakoti Peetham said once that the proper time to do sandhyavandanam is hinted in the tamil saying " Kaanaamal konaamal kandu kodu". There is one important step in Sandhyavandhanam named Argya Pradhaanam (means giving Argyam to Sun God).
As per this Tamil saying this Argya Pradhaanam is to be given along with morning Sandhyavandanam before seeing the sun (i.e. Kaanaamal) (i.e. to be performed before sunrise). So Start morning Sandhyavandanam 20 minutes before Sunrise.
The Noon Sandhyavandanam is to be when the sun is at 90 degrees (i.e. perpendicular without slanting in east side or west side but in centre up. This is called Konaamal). If the sun rises at 06:20 AM and sun sets at 06:50 PM then the Noon will be at 12:35 PM. So, start 5 minutes before Noon say at 12:30 PM.For example , if sun rises at 06:00 am and sets exactly at 06:00 PM then the Noon will be at 12:00 Noon. So, start 5 minutes before Noon say at 11:55 AM.
The evening Sandhyavandanam is to be when the sun is about to set but before it completely sets down the horizon. Kandu means that we should be able to see the sun above the horizon while doing Evening Sandhyavandanam after which the sun set would happen. If the sun set time for the day is say 06:20 PM, it is appropriate to start the evening Sandhyavandanam around 06:00 PM and complete it before the sun set.
These are all for the strict time followers in idealistic situations. Doing Sandhyavandanam without skipping is more important than adhering to the timings. So, for any reason, there happens to be slipping of the time, though one is conscious of the set scheduled timings, then also, the Sandhyavandanam has to be performed even late after giving one additional argya pradhanam for the late performing. Thus, it is emphasized by Mahaa Periyavaaa that doing three times Sandhyavandanam is more important. Anna Subramaniam of Sri RamaKrishna Math used to say that "Saappiduvadhai marandhaalum Sandhyaavandanam seivadhai marakkak koodaathu" (meaning that even we can afford to skip meals but we can not afford to skip performing Sandhya Vandanam)
In the ordinary circumstances of performing Sandhyavandanam in the aforesaid time slots punctually, We give three times argya pradhanam in morning Sandhya Vandanam. We give two times argya pradhanam in Maadhyaanika (Noon) Sandhya Vandanam. Again, it is three times argya pradhanam in evening Sandhya Vandanam. Whenever we perform late ( in delayed timings due to circumstantial reasons), then it will be for four times if it is morning, three times if it is noon and four times if it is evening.