First, the servers open up new instances automatically -- there's no cap on how many are up at one time currently. The limiter is only how many people are in the queue waiting to be deposited into the Battleground in question.
Now, here are some things to remember about the queue:
1) You need 8 people on both sides within your level range available to start an instance.
Lower levels (sub 51) will probably have a longer wait time due to the fact that most lower levels will be out... levelling. It may take an hour or more to start up in such cases depending on interest, but remember, you can play the game while in the queue.
2) Joining the queue alone using the "First Available" option will get you into the Battlegrounds more quickly than grouping and using the "Join as Group" option.
Think of it as being seated at a restaurant. If you show up with ten people, the staff must wait to seat you until there's room, which may be some time depending on the crowd. If you request a table for one, however, you'll be seated much more quickly, as there's more room for just one person than ten people that must stay together.
The same holds true for Battlegrounds. If, say, 24 players, level 51-60, are waiting to get in an instance, and they form raid groups of 6 each... not one instance will start, because none of them can fit. Unless you absolutely have to be with the people in your group, join solo. You'll be in faster because of it.
3) If you join a group that has already been placed in queue, you will NOT be placed in queue along with them.
The group leader will have to re-enter the queue in order to get everyone in the queue together again.
4) Don't forget -- you can play the game while you wait!
Roam around and play! You can even run instances while in queue -- you'll be ported straight into the Battleground when the time comes. :)
