Kristin Wiig, Jason Sudeikis, and Andy Samberg are all rumored to be on their way out at SNL. Their reps denied the report, and yet it's hard to envision any of them staying much longer. Each is in their seventh season as a performer, and each is steadily in demand for extracurricular projects; a quick survey of recent past performers' departure dates shows that seven or eight seasons seems to be the sweet spot for those with the best outside projects.
Look at the biggest success stories out of SNL over the past twenty-odd years. (You can't factor in the original cast or the Eddie Murphy years, since the producer switches led to complete cast turnovers that left the actors no choice but to leave.) Will Ferrell, Mike Myers and Dana Carvey all did seven seasons. Phil Hartman and Amy Poehler did eight apiece, while Adam Sandler was out relatively quickly in five; David Spade and Tina Fey served six (again, just counting on-camera years). But anything after eight seasons seems to be the providence of cast members who, while talented, were destined for character-actor roles, guest spots, quirky cable shows or sitcom second-banana roles as opposed to big-screen stardom: Darrell Hammond has the longest tenure, with fourteen years, while Fred Armisen is on number ten, tied with — and likely to pass — Tim Meadows. Interestingly, the current cast has three of SNL's longest-running cast members, with Armisen, Seth Meyers (on season eleven, though as head writer he has an arguably different career track) and Kenan Thompson (in his ninth season).
While there's no strict causality to this, no "if you stay past eight seasons, you will never be a major star" rule, it does point to a common trajectory: Those marked early as a standout tend to dabble in movie roles after a few seasons (supporting, and then perhaps a mediocre exploitative starring role), but still stick with the show, whether through loyalty or indentured servitude, but inevitably a big hit or a wave of offers makes staying at Studio 8H untenable, and off they go by season seven. Wiig had Bridesmaids, Sudeikis seems to show up in every third movie or in a guest role on every hip TV comedy (Eastbound & Down, It's Always Sunny in Philadelphia, Portlandia), while Samberg has Lonely Island and a very unfortunate Adam Sandler film that may well still be huge. Their reps may say they are staying put, but history will likely usher them out the door.