I cannot be sure of my answer because Japanese Language is strongly dependent by context. However, this is a possible explanation.
Actually I am sure to say that うんたらかんたら belongs to spoken Japanese. It is not an expression being used in books or serious stuff. Actually it is high probable for you to find such an expression into a manga or said by a character in the context of a Japanese anime.
It does not have a meaning, it is rather used in order to depict the situation or one's feeling. Generally you use it when:
- You are thinking to something and you are taking your time.
- You want to make a sarcastic sentence.
- You are talking with friends and want to hyperbole and put strong emphasis onto something.
Thinking to something: mumble mumble...
You can use that expression in order to point out that someone is taking time to do something.
A)ああ、ヒトミちゃん！久しぶりなんだね〜 => Aaa, Hitomi! Long time no see...
B)ケン君、久しぶりですよ！お元気？ => Ken! I guess so long time! How are you?
Yeah, fine! Last day I saw you by the station... What were 'ya doing there?
B)あの〜えとな〜なんだっけ〜 (time passes...) => Well... What did I do?... Mmmm
A)まじでうんたらかんたらね、覚えていなさそう！ヒトミちゃんオバア！ => For real, you mumbling about it. Looks like you don't remember... You grandma!
Being sarcastic or hyperbolic
うんたら alone is also used to be sarcastic or funny:
お前は１００うんたら才のオジイさんだよ！ => You are some sort of 100 years old grandpa.
In this example you can see that there is no honorific language... it is really plain... I would actually say terribly plain language, something that in real life you will never end up using. Probably in animes and mangas.
When living in Japan I did not hear people use this expression even in "easy contexts" (for examples when going to drink something together with friends). I suppose that, possibly, kids might use this expression (since I did not talk much with kids, I cannot really say whether they use such expressions or not).