I just got a new deck too and started thinking about the same thing. I don't think there's a single right answer — I usually cleanse a new deck right away, and after any heavy or emotional reading. If other people handle your cards, I cleanse them afterward.
If nothing dramatic has happened, I tend to do a gentle cleanse every few weeks or once a month. Sometimes I just shuffle while setting the intention to clear and reset. Other times I run them through moonlight, smudge them, or lay them on a selenite or quartz for a bit. Trust your gut — when the cards feel sluggish or clingy, give them a refresh.