Central Illinois plains get pretty gusty in the spring/fall. Open, empty corn fields for endless miles can allow some serious winds to get rolling.
15-20 with 25+ gusts is sucky...20-25 with 30+ gusts is plain miserable in all directions unless your with it. Above 30 sustained and I'm on the trainer.
When it's 15 or above sustained I'll keep my rides to 2hrs. Like many have said...it can be good to wear you out but it gets old quick pounding into wind that has you spinning your ass off going 13mph with gusts that will slow to to under 10 instantly. It's demoralizing.