Not long enough in my opinion...should be at least 4'x2' of floor space... for a single adult dragon.
However a small adult (18" or less) it may be fine...but hard to know if a dragon with good genetics and with the right conditions, won't get bigger than that by the time it reaches full growth. Some dragons can reach that size in 6 months and grow several inches more.
That cage will be fine to raise a dragon up to about 16-18" then move it to a bigger cage at least 4'x2' or more, but if you dont' want to make or build a bigger cage, or have the room for it, then it is best you don't get a bearded dragon till you are ready.
-----
PHLdyPayne