I see people using expressions such as "join us from 10am-2pm on January 3" or "we are going from January-May." Should "from" be there since there is no "until" or "to"?
While it's a matter of style and not all style guides give the same guidance on everything, I suspect they would in this specific matter. This is what The Chicago Manual of Style (17th ed.), 6.78, says:
The principal use of the en dash is to connect numbers and, less often, words. With continuing numbers—such as dates, times, and page numbers—it signifies up to and including (or through). For the sake of parallel construction, the word to or through (or until), never the en dash, should be used if the word from precedes the first element in such a pair; similarly, and should be used if between precedes the first element.
The years 1993–2000 were heady ones for the computer literate.
For source citations and indexing, see chapters 14–16.
In Genesis 6:13–21 we find God’s instructions to Noah.
Join us on Thursday, 11:30 a.m.–4:00 p.m., to celebrate the New Year.
I have blocked out December 2016–March 2017 to complete my manuscript.
Her articles appeared in Postwar Journal (3 November 1945–4 February 1946).
She was in college from 2012 to 2016 (not from 2012–16).
He usually naps between 11:30 a.m. and 1:30 p.m. (not between 11:30 a.m.–1:30 p.m.)
So, even though you are right that the en dash represents until or to, Chicago would have you use only one of the following two forms (rather than a hybrid):
Join us 10am–2pm on January 3.
Join from 10am to 2pm on January 3.