1) nope, not at all
2) There are a lot of variables that come into play, such as system tuning, volume level, battery condition, amp efficiency, etc. Even then, coming up with an average current draw is a guess. The HC1800 is 81 Ah. A pair of these would be 162 total Ah and should get you well within your 4-6 hour range without having to deplete the house bank too deep. Another option worth looking into would be a pair of group-29 wet cells, About 200Ah for close to the cost of a single AGM.