scene rendering and time

i see here and there ppl who time their drawing routine so that for example , the scene get drawn only every 200ms(i dont know the actual time…) is this necessary? if so, can i just use a timer?

This is not necessary and it kinda sucks to use a timer to trigger your draw. For most real-time stuff you should draw as fast as you can and if you are animating, base the animation dynamics on the elapsed time between frames. Depending on your mode your time may then be determined by the video refresh rate or some multiple thereof, and your scene complexity.