During initial plotting farmer will pick pieces of the archival history of the blockchain available at the time. The way plotting is implemented will write data sequentially as much as possible, so write amplification is very low.
As history of the blockchain is growing, we need to ensure that newly produced pieces are persisted as well. This is where replotting comes into play, ensuring both old and new pieces of the history are uniformly persisted by the network.
By the time blockchain history size doubles (some sectors expire every time segment of archival history is created), ~50% of the plot expires and needs to be re-plotted. Since over time blockchain gets bigger and bigger interval for doubling increases too and you will see replotting messages less and less frequently. It can still be quite a lot if you have a lot of space pledged.
So it is not really a function of time as such, but rather of blockchain utilization multiplied by time.