The ‘for’ is optional and doesn’t change the meaning. I’d probably leave it out. On the other hand, using ‘every’ is a little ambiguous without context and it sounds like you could be exaggerating.

Both your sentences as you have written them are grammatically correct (if you ignore the typing mistake). An alternative expression is: I work more than 8 hours every day. This is what I'd say to describe my general routine ie. what you do every day. Also, in spoken English, we often use the preposition 'for' in the example sentence you have written, eg. I work for over eight hours every day.

The first one is correct The second, although it isn't grammatically incorrect, it comes across strange linguistically