Wa alaikum assalaam wa rahmatullahi wa barakatuh,
The length of the natural medd, or medd al-asli, is two harakah, meaning the time it takes to pronounce one letter with a vowel times two. This timing is different from one reader to another, one sitting over another depending on the speed of recitation. One reading slowly will have a slower time (in terms of measurable seconds) than one reading quickly. Our basis for measuring the length of any medd of the mudood is the timing of one vowel (harakah) and then multiplied by two, four, or six. We cannot measure it in seconds and it would be incorrect to do so, since it varies according to the speed one is reading.
The timings of the ghunnah, which is part of the ikhfa', idghaam of the noon and meem saakianh and the qalb is not equal between the rules. The timings are learned from qualified teachers and passed down, but not measured in seconds, an not totally equal to each other. The early books of tajweed mention nothing about the ghunnah being two vowel counts, so we should not compare it to the mudood as it does not have the same base.
Please see: http://www.abouttajweed.com/the-ghunnah-and-its-ranks/ranks-of-the-ghunnah.html
In Hafs 'an 'Aasim by the way of Shatibiyyah, the medd wajib mutassil and medd jaa'iz munfastil are equal, either both are four vowel counts or both are five vowel counts. Both are allowed, but one must be consistent in their reading, either lengthening them both four or both five vowel counts. It is not correct in the way of Ash-Shatibiyyah for one to be longer than another. There are different turuq in tareeq At-Tayyibah which have shorter vowel counts for the medd jaai'z munfasil than the medd waajib, making it possible that a teacher is teaching one of these turuq. .
Children and other students are very good at copying their teacher, so much so, there are many children with excellent teachers who read the qur'an fluently and correctly without knowing the theory. The theory is important later, but when it comes to medd timings and ghunnah timings, reading to and listen to a qualified teacher is the main source of proper learning. We should not attribute a time length to a medd which is not consistent with the basis for the length of the mudodd: the timing of a vowel count.
May Allah grant you and those learning with you beneficial knowledge and recitation which pleases Allah.
Please also see: