Immediately. Once it leaves the hand there is no further force being exerted to accelerate the ball and air resistance will start to slow it down. 

In case

You are positioned 5 meters below the throwers hand,

the ball maybe is somewhat faster due to the

acceleration from gravity. I am not familiar with

cricket, but I think this will be more on the theoretical

side. :=)

Georg

As soon as the ball leaves the thrower's hand its horizontal velocity will start to decellerate due to air resistance. The amount that air resistance decellerates the ball depends on factors such as the ambient temperatue and pressure of the air, the condition of the surface of the ball and the velocity of the ball, however the mass of a cricket ball is great enough in comparison to its aerodynamics that the reduction in velocity over 50 metres would not be enough to notice any considerable difference in pain experienced.

If you find yourself in a position in which you are likely to get hit by a cricket ball from 50 metres away the best way to minimise pain would be to get out of the way. If you are unable to move, it could be that someone is having a laugh at your expense and you may find it more comforting to consider this than to consider your impending pain in the period during which you admire the cricket ball's trajectory as it comes to meet you.

To my knowledge no one has attempted any practical experiments to find out how much pain a cricket ball can cause at different distances, nor would I advise it (unless you are a lecturer and you wish to cull your less intelligent students).

Just out of interest, why do you ask?

I have experimental evidence for this exact scenario.    I have been hit with a indoor cricket ball on the lower part of the eye socket while batting. Firstly from about 50cm,  when the wicketkeeper attempted to throw down the stumps at the non strikers end. Then a second time from about 7m as the non striker when the batsman played a glorious straight hit that I did not even see.   Both floored me and was carried off, and there was blood all over the place.  Whilst I do not have any concrete data for this experiment, the answer is; both were very painful,  and am definitely not available to repeat this under controlled conditions for measurement.

In my experience it seems that it is the amount of pressure, not the acceleration of the ball that causes the most pain.

For example if you have a pin and press hard on the flat side of it, it does not hurt. However if you press even very gently on the sharp side, you will experience pain (DON'T TRY THIS AT HOME, my science teacher did when I was 8 to demostrate pressure).

The same applies here with the cricket ball. It depends where you get hit. Last year I got hit in the mouth by a cricket ball, and it hurt a great deal. This is because Pressure = Force/Surface Area, and the ball hit me on my teeth, which have a small surface area, therefore lots of pressure felt on the teeth, and hence lots of pain. Acceleration also has a big part in the pain felt as using Newton's 2nd Law, Force = Mass * Acceleration, therefore the greater the acceleration, the greater the force.

But why do I say the surface area of where you get hit is more important than acceleration? Well if I can draw your attention to my father, he got hit by a cricket ball at a recent ODI at Lords (http://www.youtube.com/watch?v=m5j8_IkDWGw). Now in this instance he got hit on the face, and it didn't actually hurt him that much, as the ball impacted him over a greater surface area. 

For the 10 or so years I have played cricket, it seems that people do not get hurt when hit on the forehead, but rather when they are hit on the cheek, chin ot mouth, places with less surface area.

Just one last thing I'd thought I'd mention. Horzontal velocity stays constant, so horizontally there is no acceleration. If at 5m above the ground you drop a bullet and fire a bullet from a gun at 100m/s at the same time, they both hit the ground at the same time.  

Luckily nowadays I normally catch the ball. I urge people to try the same!