From my understanding there are multiple universes which are created and destroyed continuously. Every universe arises from within the Supreme Brahman. Within each of these universes we have Gods like Brahma, Vishnu, Maheshwara. Please correct me if I am wrong here.
Assuming the above, now when does the (current) universe merge back into the Supreme Brahman?
It actually depends on what do you mean by "Universe". We should note that after day of Brahma, three worlds are merged into Supreme Brahman and after life span of Brahma, whole Brahmanda will be dissolved into Supreme Brahman.
From Srimadbhagavatam Skanda 12 Adhyay 4,
श्री-शुक उवाच
कालस् ते परमाण्व्आदिर् द्विपरार्धावधिर् नृप कथितो युगमानं च शृणु कल्प-लयाव् अपि||S'rî S'uka said: 'Time beginning with the smallest unit of the atom and culminating in the two halves [or parârdhas of the life of Brahmâ] oh King, has been described [in 3.11] together with the duration of the yugas. Now hear about the annihilation of a kalpa. (4.1)
चतुर्-युग-सहस्रं तु ब्रह्मणो दिनम् उच्यते स कल्पो यत्र मनवश्च तुर्दश विशाम्-पते||
One thousand cycles of four ages constitute a single day of Brahmā, known as a kalpa. In that period, O King, fourteen Manus come and go. (4.2)
तद्-अन्ते प्रलयस् तावान्ब्रा ह्मी रात्रिर् उदाहृता त्रयो लोका इमे तत्र कल्पन्ते प्रलयाय हि||
At their end there is a period of dissolution of the same duration that is described as the night of Brahmâ. During that time the three worlds are subject to destruction. (4.3)
एष नैमित्तिकः प्रोक्तः प्रलयो यत्र विश्व-सृक्शे तेनन्तासनो विश्वम्आत्मसात्-कृत्य चात्म-भूः||
This is said to be the periodical annihilation [or naimittika pralaya] in which [Nârâyana] the creator of the universe lies down upon His bed Ananta, to absorb the universe including Lord Brahmâ. (4.4)
द्वि-परार्धे त्व् अतिक्रान्ते ब्रह्मणः परमेष्ठिनः तदा प्रकृतयः सप्त कल्पन्ते प्रलयाय वै||
After the completion of two parârdhas [the two halves of the entire life] of the highest situated living being, Lord Brahmâ, the seven elements [mahat, ahamkâra and the five tanmâtras] are subject to destruction. (4.5)
एष प्राकृतिको राजन् प्रलयो यत्र लीयते अण्ड-कोषस् तु सङ्घातो विघात उपसादिते||
This [occasion] oh King, at which the universal egg, this aggregate [of these seven universal aspects] reaches the time of its disruption and dissolves oh King, constitutes the elemental [prâkritika] annihilation. (4.6)
All verses and translations taken from Vedabase.
Namittaka Pralaya will happen after period of 7 Manvantaras and 43 Chaturyugas (71- 28 = 43) as we live in 28th Kali Yuga of Vaivasvara Manvantara of Sweta varaha Kalpa. Vaivasvara Manvanatara is 7th Manvantara in Swetavaraha Kalpa (current Kalpa).
Prakritika Paralaya will happen after period of 50 life years of Brahma as we live in 51st year of Brahma. 360 days of Brahma (360*2 = 720 Kalpas: includes Pralaya too) constitute one year of Brahma. Srimadabagavatam 3.11.34 says
यद् अर्धम् आयुषस् तस्य परार्धम् अभिधीयते पूर्वः परार्धो ’पक्रान्तो ह्य् अपरो ’द्य प्रवर्तते||
The one hundred years of Brahmā’s life are divided into two parts, the first half and the second half. The first half of the duration of Brahmā’s life is already over, and the second half is now current.
Unfortunately, different puranas give different numbers for Swetha Varaha kalpa. For example, Skanda Purana says Swetha Varaha Kalpa is 14th Kalpa in 51st year of Brahma.
54-56. In the second Parardha of Brahma, the thirteenth Kalpa was well-known as Pitri Kalpa.
In succeding lines, Lord Shiva says Swetha Varaha Kalpa follows Pitri Kalpa. So, this makes SwethaVaraha Kalpa 14th Kalpa in 51st year of Brahma.
All Hindu mythology is in POETIC IMAGERY. Poetic Imagery is always coined to depict and express abstract things. To consider and equate them to human form, like Gods, Brahma ,Vishnu, Maheshwara, will miss the real content and purpose of Mythgology. CHANGE is the very nature of the Universe. This is depicted asCreation, Maintenance and distruction . To represent that Prakruti, in three stages is the real purpose of Trimurti. WE should not mistake them as three distinct GODS. SANSKRIT language is a Science of Sound. Every word has a Dhatu from which it springs, and has a definite meaning. BRAHMA springs from Dhatu VRIH, meaning HUGE and ever Growing. VISHNU springs from the dhatu VISH, meaning HUGE and Limitless. SHIVA springs from the dhatu, SH, meaning Auspicious. We should then imagine PARABRAHMA as that which HUGE, EVERGROWING, LIMITLESS and also AUSPICIOUS. To consider our mythology as stories similar to human stories will mislead us
