It depends on how well you care for it. Low-end estimates put the lifespan somewhere between 10-15 years, but the high-end estimate is over 100 years. Why the huge gap? Well, it comes down to largely how well you maintain your countertops.
The biggest keys of care are keeping acidic substances (e.g cleaners, vinegar, lemon juice, coffee, wine) off it as much as possible and getting it resealed every year. Daily maintenance consists of wiping it clean after use with soap and water, then wiping dry any excess water. Be sure you’re using a non-abrasive rag too, like a microfiber cloth. The stone itself won’t be breaking down anytime soon, but it may lose some of its luster over time. If this happens, you may need to bring in a professional to buff it back to a nice sheen.