If you mean the tournaments here on live chess it's not round-robin, it's swiss. This means you will play five games (or fewer if the tourney has few participants). 3-4 hours maximum.
How long does a 15|10 standard tournament take?
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Sep 27, 2014
0
#3
Thanks. I read following: https://support.chess.com/customer/portal/articles/1444837-how-do-chess-com-online-tournaments-work-
in which is states the round robin fact.
But that's not live chess tournaments, that's online aka correspondence tournaments.
You're welcome :-) good luck with your tourneys.
Sep 28, 2014
0
#7
Those live 15-10 tourneys take about 2.5 hours.
Longer than 3 hours should be rare. You play max 5 games, and each will take about 30min. Games start one after the other with no breaks, unless you have to wait because your next scheduled opponent is still playing his game. Somestimes you may draw a free round. Then you get a point and can have a break. Good luck! 
I am looking to participate in a few of these tournaments but I'm a bit concerned about the timeframe...
I read it's round robin and each round is 2 matches so in worst case it's 30min/match*2*5 is 5 hours but maybe some people can share their experience so I have a more realistic estimation :)