How quickly and how much teeth want to relapse (move) after orthodontic treatment depend on numerous factors (what they looked like before they were moved, how long after braces, etc.), but the general rule is you need retainers as long as you want to keep your teeth straight.
Even in our adult years our face and our jaws continue to grow slightly as a natural part of the aging process, which can lead the shifting and moving of teeth without retainers (even if you never had braces). Some newer orthodontic procedures involve grafting additional bone around teeth roots because this has shown to increase stability; however, for the addional cost from this procedure to be worth it there needs to be significant health (of the teeth and gums) and/or aesthetic benefits to the face, so I don't recommend it for everyone that walks in the door.
Gaps between teeth like you mentioned are another issue. when large gaps were present before braces and when teeth need to be extracted, connective fibers that normally exist between teeth (transeptal fibers) are not present and wearing retainers to hold these spaces closed becomes especially important.
Everyone is different, so how often you may need to wear your retainers (or if you need permanent fixed retention) is something you will need to experiment with on your own and with your orthodontist. If you put your retainers in after leaving them out for a night, day, week, etc. and they feel tighter than they did before your teeth are moving slightly and you will need to wear your retainers more often until you find a frequency wear the fit of your retainers stays the same.
If you break/lose your retainers and can't see your orthodontist for several days to weeks (I would be most concerned if you just got your braces off), the easiest thing to do is find any dentist or orthodontist in the area where you are and have them make a clear suck-down retainer (it looks like Invisalign trays) to hold your teeth where they are until you can see your orthodontist again.
Another important aspect to preserve the life of your retainers is to make sure you clean them with the right materials. I have a great post on this topic: http://www.freysmiles.com /blog/v.