keyframes and timelines :S

if i put 250 frames is for all blend file so i want to use 50 frames for an continuous animation so when 50 frames worked need to get 200 frames more to get to 1-50 frame again how to remove these 200 empty frames to get the 1-50 frames forever running

Just set the End time, in the Timeline window, to the length of the frames you want to loop over, in your case 50.

You can also change the Start time for looping as well. Set Start to 50 and End to 100 and you would have a 50 frame loop that starts at frame #50 and ends at frame #100.