Increasing maximum song length in DDReam and ArrowVortex
tl;dr
Download this 4GB patcher and patch the .exe of your charting editor with it.
Why does this work?
The issues DDReam and ArrowVortex have with long songs are due to running out of memory due to being 32-bit applications, which means the maximum amount of memory they can use is 2GB.
If you know a bit about addressing, you might wonder why this isn't 4 GB instead. This is because on 32-bit Windows there used to always be 2 GB reserved for the system, leaving only 2 GB maximum for user applications. The patcher linked removes this restriction so it can use the full 32-bit address space, doubling the amount of memory available.
Other notes
There's still a noticeable limit for both of these chart editors, but it's much better than what it used to be.
Waveform disappears early sometimes when loading really long songs in patched DDReam.
The 64-bit fork of ArrowVortex in development also lacks the issue with max song length, and can load songs even longer than the patched 32-bit versions due to being able to use as much memory as you have instead of being limited to 4 GB.