I've heard people say a particular Ṣalāt is not Qaza until the beginning of the next prayer. I don't believe Magrib last long and get the feeling it's gone if I delay it by 15 miniutes or so. So brothers/sisters, I would like you to please tell me how long does Magrib Ṣalāt last before it becomes Qaza.
Dusk (the time when maghrib salaah is valid) is immediately followed by night, which is when the time for eesha salaah starts. That moment of transition from dusk to night is when maghrib salaah, if not prayed, becomes qadha. There is no intermediate time between the two prayers.
Having said that, it is a sin to delay maghrib salaah till it is so late that the time of eesha is (almost) upon you.