The starting point of just about any spelljamming voyage from the Astral Plane is the local aethervane.
Using an aethervane, a skilled operator can determine the direction and speed of psychic winds, the locations and destinations of color pools, and how long they will stay open, even from a great distance. The contraptions are incredibly complicated and involve no small amount of divination magic. Calibrating, reading, and maintaining an aethervane requires vast amounts of experience and knowledge, as well as patience, a steady hand, and a keen eye.
Aethervanes vary in appearance, but they are generally about the size of a small building and consist of a complex assemblage of brass rods, weathervane-like fins, dials, gauges, and crystal lenses. Aethervanes are much too large and delicate to take on board a spelljammer and thus can only be found on waystations scattered throughout the Astral Plane.
In the Aethervane campaign, most spelljamming voyages depart from Bleak Haven, within the Astral Plane. From this point of embarkation, there are two possible types of destinations a spelljammer may have: somewhere within another plane of existence or another location in the Astral Plane.
In order to spelljam from the Astral Plane to another plane, the spelljammer must first find a color pool that leads to the target plane, preferably to a point within that plane that is fairly close to their final destination.
Most color pools lead to points within the Material Plane, which is convenient, because that is also the most common destination for spelljammers. If the voyagers are lucky, they'll find a pool that takes them directly into the Crystal Sphere that contains the world they wish to visit. Otherwise, the spelljammer will first have to leave whatever sphere the pool took them to, spend some time traveling through the Flow (typically 10-100 days), and enter the destination sphere through one of the many portals that open and close at random on its surface (or create a portal with the pass shell spell or a passage device).
Most spelljammers are capable of flight within the atmosphere of planets, or at least can take off and land through the atmosphere. In this way, they are able to approach a planet and land directly on its surface. When this is not possible, spelljamming crew may make use of smaller, more maneuverable shuttles or teleportation spells to reach the surface.
Spelljamming from Bleak Haven to another point within the Astral Plane is, of course, more straightforward and predictable. That being said, sites and objects in the Astral Plane shift position relative to each other over time, and the psychic winds can change at a moment's notice. There is no guarantee that the journey between two given locations in the Astral Plane will look the same every time.
When calculating the length of a spelljamming voyage, Dungeon Masters should keep in mind that each adventure session ideally takes no more than 23 days (the length of a month in the Bleak Haven calendar). Thus, each leg of the voyage should take no more than 12 days, assuming the characters are also expected to return to Bleak Haven by the same route. Unless the DM has a special reason to set a particular number of days to the adventure, the DM may roll 1d12 to determine how many days a given destination will take to reach. That being said, it is not necessary to specify a length of time at all if the adventure doesn't call for it. The DM can simply say that the characters spend “several days" in transit, for example, and leave it at that.
When spelljamming distances are given in terms of days, it is generally assumed that the spelljammer is traveling at top speed for only 12 hours per day. Thus, for a journey of at least 24 hours, if a spelljamming crew contains 2 viable pilots, the travel time can be essentially halved. After that, there is no benefit to including additional pilots in the crew in terms of travel time saved.