Icy,
For HR, we didn't even know all his 4 molars are out, until suddenly one day we saw his big tooth and realised that it's molar. He didn't have any fever, and didn't complain about pain at all, but he drooled.
If take my girl for example, her 2 front teeth took her about 1 month to cut (judged based on her sleeping pattern). So maybe molar will take between 1-2 months? Alternatively, you can bring him to a dentist to have a check?