I have 1 station that supplies 6 sand/second.
One that demands 5 sand/second.

In theory 2 trains could supply this, routing only takes 30 seconds, but loadind/unloading 4k sand @ 6.6 sand/second takes a whooping 600 seconds. In total the route takes ~1200 seconds.

So i've set delivery time to 1400 seconds, delivery size to 4000. That should haver enough margine. Yet, SSPP only request one train, and only once that single train is done loading and unloading, another train is requested. This mean only around 3 items/second are supplied, not the required 5. How to get it to send another train while there is already one going for this item? Both stations have a train limit of 10.

Update: A latency value of 600 on both stations fixed this.

Update: Nope, next delivery is just one train again :(

Update 3: Setting both latency to 600 and delivery time to 1200 made it dispatch 2 trains. Why this unexpected behavior?

Basically if the route takes 1200 seconds, you have to tell SSPP that. That's how it decides how many trains need to be in flight at once. I will ask, why does loading take such a long time? It sounds like you are using a mod/modpack where your trains are way bigger than your ability to load/unload. I would think that it usually wouldn't make sense to have deliveries that large if you are barely able to transfer items fast enough to keep up. Load/unload times are usually expected to be more like 30 seconds, even with only basic inserters.

Upping latency has the effect of increases the expected delivery time at a certain station, rather than globally. So effectively that will behave as if deliveries actually take up to 1800 seconds.

I'm playing Py and i'm on the early science, so the best i have is yellow inserters. Those load at around 1.1 item per second, with 6 of them that's 6.6 items per second. Some items stack up to 200 per stack, so with a wagon taking 20 stacks, that's 4000 items / 6.6 items per second, or 606 second to load a wagon.

I know this is not representative for the majority of the stations i'll be using this run, soon i'll have blue and orange inserters, and stack capacity upgrade.

However that's not my point. My current setup is half just an experiment to learn how to set things up, and it surprises me that latency is not added on top of the delivery time. The text of delivery time is "The maximum travel time from depot to provider to requester, in seconds". So i was expecting it to be just that, travel time. Then i can set up the station latency to whatever i expect the load time to be for that item. Now i need to recalculate all my travel time to include the loading time for that item also.