Is there a formal version of the term "until," used in the context of "The event will run from 8 a.m. until," signifying an indeterminate end time?
You can reword the phrase a few ways to imply no end (or an indeterminate end):
The event will begin at 8 a.m.
The event starts at 8 a.m.
You can also insert a phrase after until:
The event will run from 8 a.m. until sundown
The event will run from 8 a.m. until supplies run out
Across different days:
The event will begin each day at 8 a.m.
EDIT #2: I gave a bad answer, I apologize. I just checked again my dictionary and if you don't have a specified end time then you can use this way:
forward in time: the period from 1969 onward.
Previous answer: My Dictionary says that till is less formal than until:
USAGE In most contexts, till and until have the same meaning and are interchangeable.
The main difference is that till is generally considered to be more informal than until.
Until occurs much more frequently than till in writing. Interestingly, while it is commonly assumed that till is an abbreviated form of until (the spellings 'till and 'til reflect this), till is in fact the earlier form.
Another source confirmed that till is less formal than until.
EDIT: Sorry for the edit, but I found another entry. From the OALD (Oxford Advanced Learner's Dictionary):
Till is generally felt to be more informal than until and is used much less often in writing. At the beginning of a sentence, until is usually used.