What loop are you thinking about? Down on the west coast and up the east coast, or vice versa? Or other loops?
You can ride from the border of Thailand to Johor Baru in one day and the second day back again the other coast. But if you want to have a look at interesting places you need much more time. I spent a good month riding around and visiting interesting places in Malaysia such as Penang/Georgetown, Ipoh, Cameron Highlands (some really good motorbike roads up there) KL of course, Melakka (really interesting city), Johor Baru, Pahang, Kota Baru. And I even spent some days on the Perhentian Islands.