Learn French: L'ours dort tout l'hiver. - The bear sleeps all winter.

You can figure it out by looking at the word tout. Because tout is in its masculine singular form, we know hiver must be masculine. If it were feminine, the sentence would use toute (as in toute l'année for 'all year').

Why is it tout l'hiver and not tous l'hiver?

Tout (singular) followed by a definite article (le, la, l') means 'the whole' or 'all of'. So, tout l'hiver means 'the whole winter'. If you used the plural tous les hivers, it would completely change the meaning to 'every winter'.

Do I need a preposition like pendant (during) to say the bear sleeps 'for' the whole winter?

No, you don't need one here. Just like in English where we can simply say 'sleeps all winter', French allows you to place the time phrase tout l'hiver directly after the verb to express how long the action lasts.