When did St. Boniface become a saint?

There are several saints named Boniface. However, if you mean the Boniface called the Apostle to Germany, he was not canonized. He was proclaimed a saint by early Christians because of his martyrdom on June 5, 754. The canonization process did not come into existence until the 12th century. In the primitive Church, martyrs were immediately recognized as witnessing to the perfection of Christian life on earth, having shown the ultimate proof of their love for Christ by the offering of their lives. By the sacrifice of their lives for Christ, they attained Heaven in eternal glory and were indissolubly united to the Lord, the Head of the Mystical Body.