Usually in the morning the next day. Rarely I do two day fasts-and-long rides, but I mainly did that to see how my body would reapond. I live in the Netherlands. I ride all year long.
I think it takes weeks to get used to fasted cardio, though. At least 2-3 weeks, and I eat keto on eating days when I do that, and I ride at a leisure pace of around 25km/h despite riding on an otherwise fast, light road bike. Anything truly intense kicks me out of ketosis and quickly drives me towards starving and loosing strength. I do different things in the summer as I love a beer and french fries when the weather is nice :-)
I think it takes weeks to get used to fasted cardio, though. At least 2-3 weeks, and I eat keto on eating days when I do that, and I ride at a leisure pace of around 25km/h despite riding on an otherwise fast, light road bike. Anything truly intense kicks me out of ketosis and quickly drives me towards starving and loosing strength. I do different things in the summer as I love a beer and french fries when the weather is nice :-)