I've cleared customs in DEN on a couple of occassions. I've found that customs itself to be reasonably fast compared to other airports, but it is customs and it processing times can vary greatly. That said, even *G priority tagged, I've found myself waiting an extremely long time for luggage (+20 minutes). Luggage at DEN always seems to be slow.

My estimate is that you'll need about 30-40 minutes to get through customs. DIA is a big airport, and by the time you add in transit time through the terminals, usually a long runway taxi after landing, I'm not sure that you would make it.

That said, there are a fair amount of non-stops between DEN & LAS; although not all on UA (Frontier, Southwest, etc.) So if you miss your flight, LH/UA will be able to find a way to get you there. If I recall correctly, the LH flight gets in early-mid afternoon, so they shouldn't have a problem finding you a seat somewhere.

I'd recommend getting a later flight and try to standby for earlier.