Always "at about 2:00".
Never "about at 2:00". (And I usually don't say "never" about language possibilities, but I can't think of any time where a native English speaker might use this expression.)
There's no explicit rule that any everyday English speaker would be able to explain. It probably has to do with some unconsciously learned pattern that places "at" first to tell the listener that I'm going to say a time and then modifying the time with "about".