A Class 1 is good for 6 months, then degrades to a Class 2 for another 6 months. Even if one is not yet a PIC, it's best to get a Class 1 and never let it lapse. A former FAA Regional Chief AME told me that if some "close-call" medical condition should arise, they are more likely to renew an existing Class 1 than to upgrade a Class 2 to a Class 1.