Learn Polish: Przygotowywanie obiadu wymaga dużo czasu. - Preparing dinner requires a lot of time.

The base verb is przygotować (perfective: to finish preparing). To focus on the ongoing process, we add the suffix -ywać (a variant of -iwać used after hard consonants like 'w') to create the imperfective verb przygotowywać. The -nie ending then turns this verb into a noun meaning 'preparing'.

What is the difference between przygotowywanie and przygotowanie?

It comes down to aspect. Przygotowanie comes from the perfective verb and refers to the completed result ('the preparation'). Przygotowywanie comes from the imperfective verb and emphasizes the ongoing, time-consuming process ('the act of preparing'), which fits perfectly in a sentence about an action taking a lot of time.

Why do we use the genitive obiadu instead of the accusative obiad? Don't you 'prepare a dinner'?

Because przygotowywanie is technically a noun, not a verb! In Polish, when you use a verbal noun, the object that follows it must take the genitive case, literally translating to 'the preparation OF dinner'.

Why is czas in the genitive case (czasu)? Is it because of dużo or wymaga?

Both, actually! The quantifier dużo (a lot) always requires the noun following it to be in the genitive ('a lot of time'). However, even if you dropped dużo and just said 'it requires time', the verb wymagać (to require) is one of the special verbs in Polish that always takes its object in the genitive case.