mountain dragon

Question
How long does it take for a little juvenile mountain dragon to grow to its full size which i believe is about 20cm ?

Answer
Hi Dajana,
Just in case you are not positive on the species (tree dragon, mountain dragon, Mountain horned dragon) I am including a link where you can read the differences in each species.
http://www.anapsid.org/treedrag.html

With the smaller lizards, most reach their adult size in length between 6 and 9 months.  Of course this is based on their care. If they are not receiving proper diet, housing and one of the most important care areas, a proper source of uvb, be it natural unfiltered sunlight or manufactured lights that are of a high quality and produce uvb.
You are correct on the size.