It is normal for the hydraulic tappets to take some time to prime up and can often take two to three minutes for them all to go quiet. To ensure your oil is primed, it is always advisable to not only check that the oil light has gone out, but if you have a pressure gauge, this is a good secondary source of information. If in doubt, look inside (or remove a rocker cover) and check that oil is freely flowing from the rockers, although only a small amount is to be expected.

Getting quieter
Another good indication is that although you can still hear some, tappets, they will start to quieten down right from the first moment, so if tappet intensity quickly reduces, you can be sure that the oil has primed and safely run the engine until the last ticking stops. (normally 1 - 2 mins.).