Because the 261 has a lot of stops in Tuen Mun, expect about 45 min. Unless there's accident, there is no significant traffic on that highway. I am not a traveler on this route, so can't say exact time. KMB's internal info says around 58 minutes but that's to/from Tin Ping Estate in Sheung Shui, beyond the East Rail Station.

And I am also not sure whether it's better and faster to take minibus 44 or 44A. I can ask my friend but she's out of town for a few more days.