Now if i keep it there for a few weeks, then open it and use it and put it back in, do i need to replace the silica gel packs?
The lifespan of these packs is dependent on how much moist air they come in to contact with, not a set time scale. Once you have sealed them inside your (presumably small and airtight) container, then they will fairly rapidly absorb the moisture in the few cubic inches of contained air. If you open the container to remove your Ledger and immediately close it again, then again they will only be exposed to a few cubic inches of new moist air. If you open the container and leave it open for an hour while you use your Ledger, then they will be continually exposed to new air.
Let's say you've put in two packets which are 5 grams each. These 10 grams of silica gel will be able to absorb around 4 grams of water. At 80 degrees Fahrenheit and 100% humidity, it takes about 8000 cubic inches of air to hold that much water (
very rough calculation). So provided you are only opening it every few weeks, and provided you close it up every time and don't leave it sitting open for minutes or hours while you use your Ledger, you'll be fine to only replace the packets every few months.