There is a "phial" that sits in the main oven burner flame. This component is part of the "Flame safety device".
It looks like a small stretched pear, approximately 3cm in length with a diameter of about 0.5 cm with a thing caplilliary pipe (1mm in diameter) which looks like a thin wire coming out of one end of it
When this phial is heated up, a valve inside the flame safety device opens up and allows the full force of the gas to come through.
So when working correctly, you will initally start with a very small flame that then rises to a full flame after a short period of time.
If its taking the length of time you state its taking then there are 2 possible reasons
1) The "phial" isn't getting hot enough quickly enough which will be caused by blockages in the oven burner or the "Phial" has been knocked out of place during cleaning, eg the burner is blocked and there is no flame under it.
2) The Flame safety device is on its way out and will need replacing.
You will need a gas safe registered engineer to replace the "F.S.D"