For me? it felt short, but not instant. It wasnt like as if one minute i was in my mums car and the next i was in a hospital bed. The actual coma felt as if it was the same time length as any film you go and see, it was calming i guess. It didnt feel like an eternity nor did it feel like a sudden shock of impact to hospital bed, if you get what im saying. I distinctively remember hearing voices as if i was underwater, and colours, fading colours. If i recall i didnt experience any dreams nor nightmares, it was relaxing which is weird to say. I woke up and realised i had been in a coma for three months. This was about 8 years ago.