Since there has grown a real adversion to landing-out prevelent in the soaring community, this is a good question to hear.

Answer: totally dependant on your bird and your skills. A great experienced pilot in a kestrel can land shorter than an inexperienced newbi in a 1-26..

How to calculate: take your ship and invest in about a dozen back to back pattern tows and simulate coming in over a 50 ft obstacle, have someone note your touchdown point and your stopping point. Thats the only way to know what will be realistic for your compination of ship/experience. Be sure to practice using maximum rate of sink approach, (full spoiler and learn to slip, yes glass ships can slip).

Rule of thumb: A beginner should practice marking out a 1,000 ft minumum usable landing space. Thats a good goal to shoot for. Then as he gets better, he can get that down quite a bit shorter.