Is increasing `max_hint_window_in_ms` to days a bad idea?

I'm considering raising max_hint_window_in_ms to something like 72 hours. Anyone see issues with this? Essentially, it would allow us much longer downtime of nodes over a weekend without having to do a full repair.

It depends on the version. After C* 3.0 or DSE 5.0 when the hinted handoff storage was refactored its actually a very good idea to increase it. Before then (given your 2.1 tag assuming this is you) theres a lot of issues with accumulating too many hints highlighted in this blog post. Unless using a version after 3.0 I would not recommend increasing it too much.

To highlight some pre 3.0 issues:

• Hints are stored in a C* table and acts like a queue which is a known antipattern, builds up many tombstones and slow/expensive reads
• Hints are partitioned by node, so if one node is down a long time the partition gets very huge. This is handled better in the latest of C*/DSE but particularly in 2.1 this impacts compactions, and gcs significantly.
• Compactions are called regularly and are required, but if there is nothing getting removed this means just rewriting the mutations over and over while the node is down (wasteful)
• Individual mutations need to go through memtables and full write path vs just appended to disk