Oh my God, I hate that connection.
It is an incredibly quick way to get back to Europe from certain parts of the US, but the connection is hair-raising.
The overwhelming problem is
the freaking bus-thing :/
Like many US airports, IAD is comically out of date, a complete shambles.
You move between the bizarre terminal arrangement on these whacky overland-busses which kind of thread between the aircraft.
They come and go every 10 or 15 minutes or so, so it all depends on timing luck.
As choster explains you will almost certainly be going A->C. You DO have to use the aero-bussy device. (DON'T try the other possibilities.)
be aware you're travelling on one of the comic "partner" airlines
To avoid safety and cost regulations, actual airlines in the US use "partner" airlines for short flights. Your first flight on united "express" will be comic.
Get off the plane and run like crazy
be aware of the valet'd bag issue - this will likely determine if you make the connection or not
On your first comic flight on united "express". If you have a typical rollon "full size" carry on bag, it does not, repeat not, fit inside the aircraft.
Right at the gate, they will put it in the luggage tray of your "airplane".
When you arrive at IAD, you have to wait quite a few minutes until the (polite) ground handlers pull the "valet'd" bags out and hand them out.
This will cost you a good ten minutes.
I travel with nothing, but for this very purpose I specifically have a small carry on bag (with wheels!) which does fit inside these commuter planes, if I have to carry a few laptops or whatever.
When you get out of the first flight,
run like hell to the stupid Aero-Bussy. ask clearly where it is to get to C and don't consider any other options to get to C
pray you connect quickly with the Aero-Bussy. If you see one pulling away as you run towards it, you're probably buggered
Frankfurt inevitably seems at the far left end of C - you likely have to literally run
As with any tight connection, everything could go perfectly smoothly and you'll have no problem at all.
But missed connections are so common now in the US you might as well plan for the worst.
check on the awesome data sites about your first flight
Say your first flight is on ComicUnitedExpress100. Use the awesome data sites to examine the recent flight history of CUE100. (Simply google "recent flight history CUE100") You will learn if it tends to be late.
and finally the ultimate tip:
there is usually an earlier flight, and United will usually put you on it
carefully check regarding your first flight. Is there another one earlier?
Normally you can call United and the conversation goes like this. "As you can see I'm on 100 then 747 to frankfurt with that (insert knowing laugh) connection at IAD. Would you mind changing me to the earlier flight 69 - I just had a knee operation and I'm not able to run."
They'll normally waive the fee and change it for free (well, that's happened to me every time in similar situation with AA or United).
Of course, there may be no earlier flight or you may not be able to take it.
Wear track shoes.
Don't forget the "Will my luggage get 'valeted'?" issue, which is critical.
Finally to cheer you up remember this - you could be connecting in Chicago.