I've read that in morning the discs in spine are full of fluid and vulnerable so how much duration must I've wait after waking up to go out and play soccer without having any adverse effects? I understand a perfect time interval is difficult to decide but what would be a good enough interval?
Your discs are constantly filling up and reducing the amount of fluid inside them. The Spine Institute mentions
The water within these discs is squeezed out during the day from normal movements.
The spinal discs fill up while you're lying down at night so that they're at their fullest in the morning, therefore tighter and more vulnerable to damage.
...has made some fantastic observations on spine stiffness first thing in the morning. In a nutshell, when we lay down to sleep at night, our spine is decompressed, so the intervertebral discs actually collect water. This increased hydration status builds annular tension within the discs, and makes the spine stiffer overall. This isn't a good kind of stiffness, though; more stress is placed on the ligaments and discs than the soft tissue structures that typically protect them.
So when the discs are the fullest the stiffness can add stress to your spine.
This this LiveStrong article says:
According to the National Institute of Neurological Disorders and Stroke, 30 minutes of low-impact aerobics is ideal following long periods of inactivity.
Eric Cressey's blog mentions:
if you're someone who plans to use some of these more challenging compound movements and have to exercise in the morning, I'd encourage you to get up 30 minutes early and just focus on standing up, whether it's to read the paper, pack your lunch, or take the dog for a walk.
So it seems the general consensus is that doing around 30 minutes of light exercise / stretches would be enough to warm up, of course it will depend on your personal situation: age / existing injuries / conditioning / etc.