the Prophet said: "When one of you bows then says while he is bowing: (Subhana Rabbiyal Azim) 'Glorious is my Lord the Magnificent' three times, then he has completed his bowing. And that is the least of it. And when he prostrates and says while prostrating: (Subhana Rabbiyal A'la) 'Glorious is my Lord the Most High' three times, then he has completed his prostrations, and that is the least of it.'" (Using translation from Tirmidhī 261)   

أَنَّ رَسُولَ اللهِ ﷺ قَالَ إِذَا رَكَعَ أَحَدُكُمْ فَقَالَ سُبْحَانَ رَبِّيَ الْعَظِيمِ ثَلَاثَ مَرَّاتٍ فَقَدْ تَمَّ رُكُوعُهُ وَذَلِكَ أَدْنَاهُ وَإِذَا سَجَدَ فَقَالَ سُبْحَانَ رَبِّيَ الْأَعْلَى ثَلَاثَ مَرَّاتٍ فَقَدْ تَمَّ سُجُودُهُ وَذَلِكَ أَدْنَاهُ  

[Machine] I prayed behind the Messenger of Allah ﷺ . I asked him about the proper length of his bowing and prostration. He said, "The duration is as long as a person says 'Subhan Allah wa bihamdihi' three times."  

صَلَّيْتُ خَلْفَ رَسُولِ اللهِ ﷺ قَالَ فَسَأَلْتُهُ عَنْ قَدْرِ رُكُوعِهِ وَسُجُودِهِ فَقَالَ قَدْرُ مَا يَقُولُ الرَّجُلُ سُبْحَانَ اللهِ وَبِحَمْدِهِ ثَلَاثَ مَرَّاتٍ