If you are talking about starting up a new campaign world, it can take either a LOT of time, or very little. As a busy DM, you might honestly be best picking up a 5E AP adventure and just running that. If you don't want to do so, you might want to just buy an old copy of one of the existing settings for D&D, but they will require a little updating.
Some DMs (especially old school DMs) tend to create a "top down" approach, where they design the cosmos, deities, and general world history before detailing out the starting location and 1st adventure. This creates some vibrant, detailed settings that are often used for multiple campaigns. The downside to this is that it usually takes months to prepare, depending on how detailed you want to get it. This is probably not a good choice for a family DM, unless you are a player currently and plan to DM the next campaign.
Ed Greenwood suggests using the "bottom up" approach. You detail out the starting location (village/town/city) and 1st adventure first. You only design a half-dozen deities that are worshiped in the area, along with a few power groups (nobles, merchants, thieves' guild, etc.). Give a little thought to the surrounding area, such as topography and potential adventures. Run a couple of adventures in the starting location, while dropping hints about the surrounding areas (from rumors and news from travelers). If the players seem interested, set up an adventure in that area, and start detailing it. The is a much faster method, and can be done in only a week or so, but the downside is that your players can ask for information you don't readily have, requiring you to make it up on the fly (or just admit to the players, and tell them later).
Some DMs (especially old school DMs) tend to create a "top down" approach, where they design the cosmos, deities, and general world history before detailing out the starting location and 1st adventure. This creates some vibrant, detailed settings that are often used for multiple campaigns. The downside to this is that it usually takes months to prepare, depending on how detailed you want to get it. This is probably not a good choice for a family DM, unless you are a player currently and plan to DM the next campaign.
Ed Greenwood suggests using the "bottom up" approach. You detail out the starting location (village/town/city) and 1st adventure first. You only design a half-dozen deities that are worshiped in the area, along with a few power groups (nobles, merchants, thieves' guild, etc.). Give a little thought to the surrounding area, such as topography and potential adventures. Run a couple of adventures in the starting location, while dropping hints about the surrounding areas (from rumors and news from travelers). If the players seem interested, set up an adventure in that area, and start detailing it. The is a much faster method, and can be done in only a week or so, but the downside is that your players can ask for information you don't readily have, requiring you to make it up on the fly (or just admit to the players, and tell them later).