A few things....
when you arrive at T3, you will go directly to immigration and customs. (You can't leave the building before doing this.)
Be aware, depending on which gate your flight is assigned, the walk from the gate to immigration could 10-15 minutes.
You can probably use the egates at immigration if you are holding a US or Canadian passport. From there you take the escalators down to customs.
When you exit customs, look for signs and the ramp down to the underground walkway that will take you to the Central Bus station. You will pass the entrances to the Heathrow Express/Elizabeth line and the Underground. The Bus Terminal is actually closer to T2. But it shouldn't be more than 10 minutes once you've exited customs.
If you want some water or a snack for your bus trip, there is an M & S Simply Food on your left as you exit customs and before you head down to the underground passageway.
As for how long it will take to clear immigration.....no one knows. It depends on how many flights arrive and how many egates are open.