I change my coils after 7-10 days if I use Nautilus BVC.
But, I don't use this tank in the evenings.
But, I also never wait for the nasty taste business. Some coils I take out are still not too bad and would extend further.
If I had to guess, I'd say 5-8 days with an average gunky liquid. I also run them 'high' so running them low with a clean liquid could double that.
I'm finding the Cubis coils noticeably longer-lasting. Maybe progress is being made.