In a previous sentence I think it was "i vackan". Can you use "i" or "om", or are there specific rules?
I wouldn't say "åtta timmar i dagen" or "åtta timmar om veckan". Actually, I'd rather say "åtta timmar per dag" than this sentence here, but this is also correct. With weeks, you have to say "i veckan" or "per vecka".
So would you say I have a greater chance of being correct/sounding natural if I just stick to "per"?
At least when talking to me :-) It might be that other native speakers have better rules for this...
Seems a lot like German. You can say "am Tag" or "pro Tag" but not "im Tag" (day). Likewise, you can say "in der Woche" or "pro Woche", but not "an der Woche" (week).
According to Google Translate, you use "i" when talking about seconds, minutes, hours, weeks, and months. You use "om" when talking about days and years. Does that sound correct?
From what i learnt so far i + [ span of time, definite form] = per + [span of time, indefinite form] = om + [span of time, definite form];
Examples: i veckan = per vecka = om veckan = per week.
Of course it would be nice to hear a mod on this.