Hey Givemewings,
This is because the system works in such a manner as that if it can't give you the first day in the string off, then it throws out the hole string. So, always bid for your most important day off first, but also try it in reverse order as well. Sometimes, or most of the time, you may get part of the string if not all of it. For example, if you have leave on say the 10 to the 15th of the month, and you want off 5 days before, always put the bid in as award the 9th-5th, not the 5th-9th.
To answer the original schedule questions, I believe the number is 7 days off, but I would not swear to that as those rules seem to change each bid session. In any case, because the system uses points, always put a date string greater than you think you will be awarded. For example, if you want 7 days off, but only think you will get 4, put the bid in for 7 days and see what happens (But BE SURE TO PUT THE MOST IMPORTANT DAY YOU NEED FIRST IN THE STRING!!!!) You never know, you might get surprised. Remember, the system is a computer and it only really has 2 goals. First and foremost, build you a schedule into the company set criteria for hours. Number two, honor as many of your bids as possible while building said schedule in order to award you the highest possible point total.
Hope this helps a bit with your bid or to whoever is reading. Good luck with your bid, i hope you get off the days you need.