In big companies, doing it by tenure with the company is common. A typical schedule is something like: start at 2 weeks/yr, and get an extra week/yr for every 5 years you stay with the company, topping out at 6 weeks/yr. That model has run into trouble as people change jobs more, and is uncommon in the Valley where people change jobs even more than that. Some companies have ditched it, while others will negotiate giving you credit for past jobs, e.g. I believe if Exxon hires you away from a 20-year career with BP, they'll often give you equivalent seniority at Exxon for benefits purposes.