When will a Democrat next win a statewide race in Texas?
It would be based on circumstances.
If one is thinking U.S. Senate: It would likely occur in a midterm election cycle in which the president of the United States is affiliated with the Republican Party and the Democrats flip a U.S. Senate seat in Texas.
If one is thinking U.S. President: It would likely occur in the next applicable U.S. presidential election in which the White House is in the column for the Republicans and it ends up switching to the Democrats. Under those circumstances, one of the pickup states would be Texas.