Keep until the rest of the stuff gets put away, or if they come after that (I know an increasing number of folks who send New Year cards) I keep them for about a week before pitching.
I keep the ones that I consider extra special. The rest I donate to St Jude’s. They take used Christmas cards & give the children projects where they remove the backs & then replace them with nice clean backs. Then the new cards are packaged & sold in their gift shop the following Christmas to help raise money for their research & treatments. It helps me to feel that I’m giving back!!!