Different larvae have different life spans. You can use a simple guide like this one:

http://www.penobscotswcd.org/publication...

to identify what your larvae are. From there, you can look up the life cycle of the species to see how long it will take to become an adult. The big guy in the center there looks like a dragonfly nymph.

There are other guides out there, of course, if you don't like the one linked. Just search on "aquatic insect identification" for lists of others.

I was looking in my pond today and found loads of strange bugs and I think they're diving beetle larvae, if they are when will they become adults? and if not what are they?

two of them are in the picture I've attached.