The standard shape is four years with a one-year cliff. Nothing vests for the first twelve months; at the one-year mark 25% vests in a lump; the remainder vests monthly over the following three years. Leave before the cliff and you keep nothing. Leave at two and a half years and you keep roughly 62.5%. The unvested portion returns to the company.
It matters most for the situation nobody plans for: a co-founder leaving early. Without vesting, someone who spends four months on the business and then disappears keeps half of it forever, and every future investor sees a dead 50% on the cap table — often a deal-breaker. That is why founders vest their own shares too. It feels absurd to earn equity in your own company until the first time a partnership ends badly, at which point it is the clause that saved the business.
A concrete example: you and a co-founder each take 4,000,000 shares on a four-year monthly schedule with a one-year cliff. Fourteen months in, they take a full-time job elsewhere. They have vested 25% at the cliff plus two further months — about 1,166,000 shares, or roughly 29% of their grant. The other 2,834,000 return to the company and are available for the person you hire to replace them. Without the schedule, they would simply own half of everything you build over the next decade.