I live in a traditional 3 bedroom semi.
7 to 10-feet higher/longer than the height of the gutter or other leaning/landing point. This additional height/length is needed for the ladder's bottom being brought away from the building, so the ladder is pitched at a safe and comfortable angle.
Go with 10-feet additional, if you plan on getting onto the roof. And don't just buy any ladder. If you're a fatty, you'll need to be very considerate of the ladder's weight/load capacity. And, if the yard slopes, you'll need add-on leveling legs. Other add-ons are available too.