Recently, Peter uttered:

I have a little - 2' 6" approx - oak tree in a 18" pot. How long can I keep
it? If you read this my other post about compost should say compost smelling
of silage

Check out some Bonsai websites.
This is a good forum:
http://tinyurl.com/9m8v

You can keep it in a pot for as long as you like, and the tree will be
perfectly happy as long as you look after it. You will have to cut the tap
root after a few years though. If you want to grow it on to a full size
specimen, get it in the ground ASAP !!
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It's all on http://support.microsoft.com/ somewhere....
If you can be bothered to look for it....