I was there on Friday of last week - very impressive collection and only 60 cents entry fee. Opening hours are 10am to 2pm.
I was staying at Barajas Airport and it was easy to get there using the excellent metro and bus networks. Line 8 from Barajas to Neiuvos Ministeros then line 10 to Principe Pio then bus 525 (or any number) to Mostoles. Ask driver for 'Museo De aire' and the bus stop is close to a footbridge over the motorway which leads to the museum. Journey time was 45 mins although could be longer in the rush hours (I started at 9am ish) There is a metro stop at Cuatro Vientos but this is a long way (2K+) from the museum although you could probably get a bus from there.
Have put the pictures I took on the Key publishing website.