Tex. Elec. Code § 33.052

A watcher at a precinct polling place may begin service at any time after the presiding judge arrives at the polling place on election day and may remain at the polling place until the presiding judge and the clerks complete their duties there. A watcher may serve at the polling place during the hours the watcher chooses, except that if the watcher is present at the polling place when ballots are counted, the watcher may not leave until the counting is complete.