The length of time it takes to become a certified welder will depend on whether you're being certified by an employer or a third-party organization.
Employer certification is usually specific to single welding process. If you have experience tackling that process, certification can be completed with no need for additional training. This type of certification can be completed in as little as a couple of weeks.
Third-party certification by an organization, such as the American Welding Society, tends to be more comprehensive than employer certification, and it requires you to pass both written and practical tests. Vocational schools offer training that assist with this kind of certification, and it can take anywhere from seven months to two years to complete a program that prepares you to take the tests. You can also choose to study on your own, and study guides and practice tests are available online. The American Welding Society will send your welder certification card within four to five weeks after you've passed the tests.