For me, it's whatever I believe would take less time. I can write a left-pad function in less time than it would take to find the npm package, install it, import it and integrate it.

With more complex systems, how much integration/customisation is needed vs building something yourself?

Often I'll look for a library, find a few candidates that don't really suit well and 'resort' to committing to writing something myself. Similar to planning a project, it can take time to work out how much time something is going to take.