I usually keep 'em about a year or so. Some lasted a little longer.
However, I consider Yamaha basically the same keyboard now as it was with the PSR2000, with minor upgrades on each model.
When arrangers first started, they improved quite dramatically between models and companies; not so much or so fast now.
I generally buy mine fairly cheap and sell 'em while I can still not lost much, if anything.
I've had E50 for about four or five years, but it's basically my backup now.
BTW, BK7m is quite remarkable once you set it up. Excellent sounds, including drums. I used Edirol controller and it has a program to help map the commands. You can do a lot with the controller than can not be done directly from the module.
DonM