Respected Gopalan sir,
With reference to the 15 minutes which you have mentioned for gayatri Mantram , i would like to know how many times Gayathri can be recited . The reason behind my query is simply borne by the fact that as per our Veda Shastrams Gayatri Mahamantram should be recited slowly with proper pronunciation , by this I mean that the mantram should be split in to 5 parts and recited mentally with a pause after ever aksharm ( Actually the mantram should be recited with proper rhytm). Out of my personal experience I can vouch for the fact that it takes me approximately 4.5 to 5 minutes to recite 12 gayathris . Again if you follow the same procedure while doing Pranayam ( you recite Gayathri in that also) and also while giving aryghyams , i beleive performing Sandhyavandanam will take minimum 30 -35 minutes assuming one recites gayathri 24 times - slowly with pause and through correct pronunciation.
Kind regards
Narayan