Within the book Nitya Karma Pūjā Prakāśa published by Gita Press Gorakhpur, the following statements quoted from nibandha texts.
गायत्रीं दशधा जप्त्वा सन्ध्यायाः फलमाप्नुयात् स्मृतिसमुच्चय
One earns the benefit of having performed sandhyavandanā by performing the japa of Gāyatrī 10 times.
आपन्नश्चाशुचिः काले तिष्ठन्नपि जपेद्दश आचारभूषण
Even in times of distress & pollution, one must perform the japa (of Gāyatrī) 10 times.
Manusmṛti defines distress as the time when a person is unable to maintain himself & his family by performing the occupation sanctioned for him on the basis of his caste-status while pollution is defined as that caused by birth (sūtaka) & death (mṛtaka).
The time for performing sandhyāvandanā is defined as
Prātaḥsandhyā is to be done early in the morning while the stars are visible. When the sun is in midle of the sky, the madhyāhnasandhyā is to be performed; & the sayaṁsandhyā is to be performed while the sun is visibly going down. Now again, the following distinctions are made regarding the time of performing sandhyāvandanā - prātaḥsandhyā performed while the stars are still visible is the best; when the stars have disappeared (i.e dawn), middling; and with the sun risen above the horizon, inferior. Similarly the sayaṁsandhyā is of 3 kinds - best, middling, and inferior. Performance of sayaṁsandhyā while the sun is visibly disappearing is the best; when the sun has gone down the horizon (i.e dusk), it is middling and when the stars have become visible, it is inferior....At the sunrise and the sunset the time for sandhyāvandanā is 2 daṇḍas (i.e 48 minutes) and if sandhyāvandanā be not done or rather neglected in the interval, prāyaśchitta is to be duly performed. [Devībhāgavata Purāṇa:9:16]
The prāyaśchitta for failing to perform sandhyāvandanā is defined as
सर्वथा सन्ध्यालोपे प्रतिसन्ध्येमेकोपवासोऽयुतमष्टोत्तरसहस्रं वा गायत्रीजपः । अत्यशक्तौ प्रतिसन्ध्यालोपं शतगायत्री जपः । द्व्यहं त्र्यहं लोपे तदावृत्तिः । ततः परं कृच्छ्रादिकल्प्यम् । धर्मसिन्धु
In case of non-performance of sandhyāvandanā, one must fast or perform the japa of Gāyatrī 10,000 or 1008 times for each time of the day the sandhyāvandanā is missed. Extremely incapable persons will perform the japa of Gāyatrī 100 times for ommision of sandhyāvandanā on each time of the day. The same is to be repeated in case of ommision for 2 or 3 consecutive days. Beyond that, one is to resort to the likes of Kṛcchra(vrata).
The Devībhāgavata Purāṇa mandates the japa of Gāyatrī 108 times before the performance of sandhyāvandanā for that time of the day if the same for the previous time had been ommitted (For example - if sayaṁsandhyā has been ommitted, then Gāyatrījapa is to be performed 108 times before starting prātaḥsandhyā)
(N.B - Nibandha refers to a class of Hindu literature which is mainly a compilation of opinions extracted by religious scholars from smṛti, purāṇa, pariśiṣṭha, gṛhyasūtra, dharmasūtra & other pre-existing nibandha texts on a list of topics related to personal law, social conduct & religious observances from which the scholars determined which opinion is to be followed. These texts are considered more authorative than the original smṛti texts in modern times because they quote texts which were lost to time & lines attributed to smṛti texts but not found in the printed version of those very same texts, thereby providing evidence of interpolation. For example, the practice of wearing maṅgalasūtra is attributed by nibandhakāras to Śaunakasmṛti although manuscripts of Śaunakasmṛti itself hasn't been found till date. Divergence of opinion among the nibandhakāras themselves results in variance in the observance of Hindu customs. For example - marrying one's maternal cousin is a popular practice among South Indian Hindus due to its practical applications & the fact that it follows the prohibition on marrying within one's own gotra but this practice is abhorrent to North Indian Hindus)
