Under the present USFA system, results as old as 4 years may affect your rating. Under the FIE system, only 1-yearold results come into play.
The longer the time period is, the less risk is that variations in competitive frequency will affect ratings. OTOH, a long time period for time-out will lead to a single out-of-character good result will overrate people, and any degradation in fencing strength will take a long time to show in ratings.
A too short time period will obviously have the opposite problems, too much emphasis on competing frequently and a single bad month or quarter leading to a fencer being seriously underrated.
So, what is the best compromise for a rating system catering to fencers which are not complete beginners, nor have any real chance of making it to the national teams?
Have a nice time!