It's up to you... Try a range of 50 feet to 100 feet, see what you like.
The shorter the lines, the faster the kite and less sky you have to fly across... But everything is more responsive, and better for tricks.
The longer the lines, the slower the kite will seem and you'll have more sky to cover... Making it more ideal for flying shapes and executing precise manuevers.
Always feel free to ask more questions, kite fliers are a chatty bunch.