In MTG ARENA, Tournament Challenges give both players 30 minutes to complete their match. This timer is specific to each player, and only ticks down when the player has priority or a decision to make. If a player’s timer runs out, that player is assumed to have lost that game and any remaining games in that match (if necessary).
The Lead Admin of your online tournament may ask Judges to reach out to players in matches that have not reported results after 50 minutes to confirm if they are still playing. Failure to respond may result in penalties. (See: Magic Digital IPG 3.5 – Disconnects)