How can you appropriately use “延长”？
The usage 我们的会议延长了两个小时 is correct. When you want to say something was extended by [the extra time period], you use 延长(了)[time period]. If you want to say till what time something will be extended to, you use 延长到[time/date] or 延长至[time/date].

因运动员旧伤复发需要及时治疗，球赛将延长一个小时。
Due to the athlete's suffering from an old injury that requires timely treatment, the match will be extended by one hour.

Correction: boarding time is postponed rather than extended. Notice the Chinese word for postpone is 延后 not 延长.

飞机误点了，登机时间将延后至下午三点。
The plane is late: the boarding time will be postponed to 3pm.