We are "all cottages" including a 5-bedroom, 2-bath farmhouse that sleeps up to 10 (~1834 square feet), a 6-bedroom, 3-bath lodge that sleeps up to 9 (~1676 square feet), plus seven other cottages. All have complete kitchens.
We don't serve breakfast, and we do not clean or make beds during occupancy, but guests can exchange their sheets and towels for fresh ones if they wish by bringing them to our linen shed. So in many ways we are really more akin to "vacation rentals" than "B&B."
Morticia raises the issue of cleaning. We estimate that it should take about 5 person-hours to clean and changeover the farmhouse and the lodge, and 2 to 3 person-hours to clean and changeover each of the cottages (depending on the number of beds and bathrooms in each). We hire a crew of cleaners every year, so we personally do a minimum amount of cleaning. Pay your cleaners well if you want to get good cleaners who will be reliable and come back year after year (in our seasonal area, that's $20 to $25 per hour). We send all our laundry out to a service, and we estimate the laundry cost for a change-over at about $5/person max occupancy (so the farmhouse runs about $50 and the lodge $45...)
So, changing over these houses is expensive, and we set a three-night minimum in the off-season and a full week minimum during peak season, to make sure we are generating enough revenue to justify the costs.
From a business perspective, the question to ask is this: Is your area, or perhaps your specific location (even down to this particular house!), enough of a "destination" that guests are likely to want to come and stay for three nights or a week or even longer? Think also about the types of guests you are targeting with your business -- Are you likely to attract families or groups who would want this kind of accommodation, or do you see yourself more serving couples?
If you are thinking about nightly turnovers, how much would you have to charge to cover your costs, and will the market bear that? The kitchen is a potential problem area here. For nightly turnovers, you would probably want to take the kitchen out of the house to reduce the cleaning costs, but for longer term "vacation" stays, having the kitchen would be an important draw....
The point here is, that unless you just have money to burn, you need to make a business case for acquiring this house in addition to the "inn" you are looking at..