The seas that surround Heluso & Milonda have definite patterns in their winds and currents. These contribute directly to how goods and people can move from one continent to another.
The Great Channel
Generally the winds across the sea between the continents blows west to east and the currents go north to south. This makes sailing to Milonda from Heluso relatively easy. The most travelled route is known as the Great Channel. As traditionally delineated, the Channel starts ships just south of the Ob-Lob city in Broadland, passes near the Inuan islands and they arrive at Milonda in a couple of weeks. Easy. As long as you can stay away from pirates.
Coming back is trickier. To come back the same way you're fighting the prevailing wind the whole time. Most ships do really poorly at that, though some Ob-Lobs do just muscle through (taking about a month to do it - up to three if you're not sailing an Ob-Lob ship). Fighting the wind like this makes most ships coming through this way easy pickings for pirates, so it's not most Captains' first choice.
The Sea of Madness
Just to the south of the main west-east trade corridor is an unpredictable region. Some Helusan sailors have called it the Sea of Madness, basically because of the ever-shifting nature of the winds and currents. Ships can get becalmed for months, while watching storms pass them by. Ob-Lob captains seem to have some way of getting through these waters relatively unworried. If you know the route, the return trip takes two to three weeks. If you go in blind, you're very likely going to die there.
The Shadow Sea
The other direct option is to take the Shadow Sea. This lies to the north of the main trade corridor and beneath/above the Sunless Plains. This is only slightly easier than fighting the Prevailing winds and is made worse by treacherous reefs and rumours of horrible creatures living beneath the surface. Superstitious (read: most) sailors dislike sailing the Shadow Sea unless it's absolutely necessary. It only takes about six weeks to take this route. Most of that time is spent coddling sailors. Or repairing the damage done by sea serpents. Either way.
The Northern Passage
The longest route back from Milonda goes north along the Milondese coast until the wind shifts east to west. This happens north of where the continent meets the sea. This wind can take ships back to Heluso quickly, passing Freeport as they go. It takes about 4 times longer than returning by the Sea of Madness (assuming you know the route), so plan for about three relatively safe and boring months. The thing that takes so long in this case is sailing up the Milondese coast. The winds are generally favourable and the currents aren't too bad; it's just a long way to go. If you are an Ob-Lob you've got much better ways to get where you're going so you generally leave this route alone. If you're not an Ob-Lob this is the most popular route.
The Icy Sea
There's also the Icy Sea route, which is a comparative backwater. It's often faster and safer to ship things to and from the Truil Wastes by sea. Even taking into consideration the paucity of ports in that savage land. And the fact that gathering enough Truils together to get anything out of them worth financing an expedition for can take almost as long as taking a caravan across the continent. The profit on these routes aren't great, but it can keep some companies going, especially if they can get a good batch of rarities in one go.
