The finish time of Opposition day debates is fixed at the usual finish time for main business of 10pm on Mondays, 7pm on Tuesdays and Wednesdays, and 5pm on Thursdays.
Start times are more variable, depending on two things. First, Opposition days begin after other business to be dealt with that day is finished, including Question Time, and any urgent questions or oral statements.
Secondly, the Opposition may decide to use the time available for a single debate, or for two debate on different subjects. When there are two subjects, the time is roughly split, with the first typically ending at approximately 7pm on Mondays, 4pm on Tuesdays or Wednesdays, and 2pm on Thursdays (or earlier, if there are no more MPs to speak), and the second following on.
There can be a vote on the first debate. Votes take about 15 minutes each, so one or two of them will reduce the time available for the second debate, which has a fixed finish time.
Occasionally, there are Opposition half-days. In these cases, a Business of the House motion agreed in advance usually sets out how long the debate can last. It’s typically three hours.