Some igneous rock never become metamorphic - the ones that do, however, are usually rocks that were created earlier in the Earth’s history, and were covered, squeezed, and/or baked (by other, later igneous rocks), and from the cumulative heat and pressure for perhaps millions of years (or longer). read more
Depends what you consider to be a "long time". It is believed metamorphic rock can form in as little as 100,000 years. read more
Metamorphic rocks (ones formed when any type of rock, including older metamorphic rocks are heated greater than 200 degrees centigrade and are exerted to over 1500 bars of pressure) tend to differ quite a lot in formation times, some take hundreds of thousands of years, while some take only a few thousand. read more