As soon as possible. The amount of time it takes to interview and process a visa varies greatly. Estimated wait and processing times can be found here: https://travel.state.gov/content/travel/en/us-visas/visa-information-resources/wait-times.html
If you are applying for an O or P visa, you must first have your petition approved by USCIS (see here: https://travel.state.gov/content/travel/en/us-visas/employment/temporary-worker-visas.html). Petition approval may take as long as 180 days.
Please let SXSW know how you intend to enter the United States (whether using the VWP or a visa) by February 4, 2019, to secure your spot in the SXSW Music Festival. This information can be updated within Artist Admin by visiting the Members section and editing each listed Member and Crew Member information. For questions contact email@example.com.
For more information about obtaining a visa or estimated timeframes, a list of attorneys and agencies is provided in the Additional Resources FAQ.