Actually, most campaigns send out a collection team in the day after election day to take down the big sign as well as signs put up on public property. They also typically pick them up from lawns as requested.

Some will wait a day or two to celebrate the win but sign pickup

Most candidates keep the signs from one campaign to another. It takes a while for new signs to be printed at the beginning of a campaign. So, using old signs means getting signs up in the early days before your opponents and saving costs.