So, I don't recall it taking that long. Of course you are at the mercy of how often the busses and Metro runs. From the hotel, you walk over a footbridge over the highway pretty much right at the hotel and catch the bus to Canillejas (I think) and pick up the green line. The front desk will certainly be able to tell you and also give you change for the bus.

On the way back, the bus stops right at the hotel, but a) I don't know how late it runs (once again, ask the hotel staff), and b) make sure you get on the right bus.