"How much do dogs sleep on average?"
Translation:Hur mycket sover hundar i genomsnitt?
You're right that since it's a question, the V2 rule does not apply. In questions, the rule is that the verb needs to go before the subject. If there are adverbs that modify the verb, they need to go after the verb. However if there's a noun that the question is asking about, it will go before:
Hur många invånare har Sverige? 'How many citizens does Sweden have?'
So it's wrong to say Hur mycket i genomsnitt sover hundar?, but you would say Hur mycket kött äter hundar i genomsnitt? ('How much meat do dogs eat on average?')