I have flown this very flight on Capital Airlines PEK-ERL for the same purpose: resetting my visa duration of stay clock. When I went, flight was an early departure about 7:30 arr ERL 8:45. The entire round trip can be done in 24 hours Beijing-to-Beijing using ground transport to return. The crossing and recrossing sequence takes about 3-ish hours. If you want to fly back to Beijing, you'll have to overnight at Erlian and get the return flight to PEK the next morning.

Caveat: if you just want to hop the border without any particular visa reset need, don't bother. Zamyn Uud on the Mongolian side is a dusty and uninspiring frontier town and there's nothing worth seeing.

If you need a full process description of how to do the visa run, start a new thread just for that, and we'll leave this for Capital Airlines. Capital is part of the Hainan Airlines family and has some funky but useful destinations. I have no problem flying them.