I'll be at my office by 7 p.m. I'll be at my office until 7 p.m

"By 7 p.m." means that you'll arrive at the office at some point before 7 p.m., so that at 7 p.m. you'll certainly be there - this is usually intended as a promise that you won't be late to arrive. It doesn't mean you won't be there earlier.

"Until 7 p.m." means that you won't leave the office before 7 p.m.

Also, at the office (as opposed to in the office) doesn't necessarily mean you have to be physically stuck in the room - if you take a bathroom break, for example, or go outside for a smoke you can still say you're at the office even if you're not physically in it.