From Lippert's manual. They do not state how long the jacks can be down. Myself, if I keep my jacks down I exercise them every now and again just to keep the seals lubricated and this also causes me to exercise the slides as well. Moving parts that are stationary for long periods of time seem to break.
If jacks are down for extended periods, it is recommended to spray exposed leveling jack rods with a silicone lubricant every seven days for protection. If your coach is located in a salty environment, it is recommended to spray the rods every 2 to 3 days.
US Army (Ret)
2020 Entegra Accolade 37TS
2019 Jeep Grand Cherokee Trailhawk (Toad)
FMCA - F432054