Schedugram has a caption limit (also for 'first comments') because we know that posts with longer captions have a harder time going up, or they fail. When we originally didn't have any limits in place, this caused a lot of complaints from users whose posts didn't go up.
Our caption limit is 750 characters. This is long enough for 99.999% of our customers – and for most people who are reading Instagram; after all, it's not a blogging platform.
We see these errors for a range of reasons. Sometimes it's related to our keyboard, but we also see a higher error rate from Instagram (what we call "upload errors") from accounts that frequently post long captions. Unfortunately a lot of this is out of our control (even our keyboard which is a bit of a workaround to give you Emoji and most of the normal 'set' of letters we can only do so much with!).
One thing that we do see is that captions in Arabic or that have a lot of similarly complex characters do seem to need to be shorter – this one does appear to be more on our keyboard's side, and customers with Arabic captions (or similarly complex character-filled captions, but we do see it a lot more with Arabic posts) should keep to a shorter limit (300-400 characters).
If the caption is too long (even when it's within our limits) – the post will fail, and you'll get an email saying that it failed. If this happens, please make sure you shorten down your caption length to avoid it happening in the future!