We let everyone book it up to 90 minutes every day. They can book it for longer periods but then I charge them 50% off the daily rate of $75 for half a day, $100 for a full day. Most meetings are only 90 minutes long or shorter so I rarely have to charge or monitor the conference room. The limit is really in place so I can stop people from abusing it and booking it all day.
We don't have many issues with the conference room because we have one phone booth for every 15 members. This keeps people from using the conference room as a telephone room, which is very common.