“Dw i wedi bod yn dysgu Cymraeg am biti mis”
Although, I’m not entirely sure if that is how it’s spelt, or if it actually has an official spelling - could be ‘bitu’ or 'buty/buti, or ‘butu’…
The word about is what I’m after. App geiriaduron has: o boptu as an option. But that isn’t what I’m hearing. I’m keen to know what I’m hearing.
I’d write it the way Gavin did. This is what Iestyn had to say about the issue in another thread:-
"I’m not sure that ayone really knows how it’s spelt, Don! I write it “biti” in the transcript for the listening practice after lesson 5, but it depends on the individual speaker how it sounds exactly as well.
It comes from “o bob tu” (on every side of), and can be biti, bwytu, bythdu, bithdu…
When it’s written together with the “am” as one word, I commonly see it spelled as ambytu or amboutu.
Not that it is important, but the GPC gives it’s derivation as follows.
“Peu-” seems to be a (rare) prefix meaning “every”. [ eg- “Peunydd” is a form of “beunydd”, “every day”, as in “bara beunyddiol”, “daily bread”.]
This gave “peu+tu”, “Peutu”, a word meaning every side.
This leads to various forms derived from “am beutu” or “o beutu” as meaning “on every side, around, about”, leading to the use of it to mean “about/around as in approximately” (perhaps under influence of the similar sound and meaning of the English “about”)
There are, as everyone else has pointed out, many many ways you will hear it said and see it written (though for what little it is worth, my experience of the written form agrees with Kinetic’s.)
But as Huw pointed out, Iestyn says he would spell the phrase “am biti” (if he had to!), so that is what he is saying, and it is what you are hearing!
Thanks everyone that helps heaps. I seem to need to understand these things. I’m the same with English. I like to know word derivations and how they work. I would have been a linguist in another life. It is still a little odd to me how many variations of spelling the Welsh language has. Pronunciation, I understand, there are huge variations in how English speakers pronounce words but, correct me if I’m wrong, English spelling is more standardised. I’m not saying this is good or bad. It’s just what I’m used to. Certainly for English language learners confronting words that are so non phonetic is a challenge. In Welsh, they just seem to write words how they are said, with manifold variations. 
Welsh spelling is very phonetic. English spelling has one spelling, which bears little connection to how the word is pronounced!
Even the difference between “literary” Welsh and “spoken” Welsh has far less difference to the almost random spelling in English.
“Amboutu/byti/bythi/obiti” etcetera etcetera (in the voice of Yul Brinner) is in some ways a special case. Particularly riven with different forms. Though the are many dialects, many ways of pronouncing the words, they are (in my view) always closer to the pronunciation of “Welsh as she is spelled” than English is.
Other peoples views may vary 
The thing is, because Welsh is so phonetic, a tradition has sprung up that you can simply spell the language how you speak it. Thus, in letters, and now texts, twitters, blogs, whatever, the written language is simply a written form of how you speak it.
Rather off putting to someone from a different tradition, perhaps - but rather nice and logical, in my view!
U r rite, Owain
Yes, I’m getting used to it. A radically different approach to English but…pam lai? It’s a different language.
The interweb is a wonderful thing: http://books.google.com.au/books?id=VA0NAQAAIAAJ&pg=PA22&lpg=PA22&dq=welsh+peun&source=bl&ots=-CcizAUvUk&sig=GZfc4Byz1sMeybUB0RTwcDqXKuE&hl=en&sa=X&ei=7IjxU7fEGY_t8AXyzIGQCw&ved=0CEoQ6AEwBg#v=onepage&q=welsh%20peun&f=false
Words like peunyd(d) (every day), peunoeth (every night) have ‘peun’ (pobn) as the accusative from of of ‘pob’, And the ‘byn’ in ‘erbyn’ is the accusative form of ‘pen’. Who’d have thunk it.
Ooo, interesting! Then apparently “by a wrong separation peu-, was abstracted from the old peunydd, peunoeth, and transferred to various other nouns, denoting " every”. "
https://archive.org/stream/ycymmrodor08cymmuoft/ycymmrodor08cymmuoft_djvu.txt
Thanks for that link! The only “old case survival” I knew of was in the Place name “Cardiff”.
Caer + Tâf (the Castle of the (river) Tâf would just be “Caerdâf” nowadays, but when it was first named way back when, the genetive form of “Tâf” was “Tyf”, so it became “Caerdyf”.
(Then, a common f>dd change occurred later on, giving “Caerdydd”)
As you say, astoundingly interesting!
Louis/Owain - awesome, thanks! I’d heard of the “Caerdyf” thing before, but the peu- and erbyn things are new ones on me and fascinating. Keep 'em coming 
Seeing you asked for it…
Remnants of neuter nouns!
http://en.m.wikisource.org/wiki/A_Welsh_Grammar,_Historical_and_Comparative/Accidence1
The following traces of the old neuter survive : (i) nouns of vacilla- ting gender 142 i. (2) The neut. dual in Kelt, had been reformed with -n on the analogy of the sing., e.g. Ir. da n-droch ’ 2 wheels’; hence in W. after dau, some nouns, originally neuter, keep p-, t-, c- unmutated 106 iii (4) ; thus dau cant or deucant ’ 200 ', dau tu or deutu ’ both sides ’ ; and by analogy dau pen or deupen ’ two ends '.
Just wondering… What was the rationale for changing from the mae ffrind gyda fi pattern to the mae gyda fi ffrind? It is doing my head in. Though, I think I actually prefer the new pattern.
I am enjoying the new course. I think it will be easier to use with learner’s groups as there are more patterns to practice.
I think this came up in another thread, and it meant you could use “Mae dal gyda fi ffrind” in the same pattern. Don’t take my word for that, though.
Edit: Here we go: https://www.saysomethingin.com/welsh/forum/0057c8b07c6a3d15?query=
That said, you can of course use mae ffrind dal gyda fi. The word order doesn’t matter too much.
Thanks. Got it. I think Iestyn mentioned in the first course that it is sometimes the other way round. I’d never actually seen or heard it until now. It makes sense with the dal.