Several hours is good. Saddle time is key, and you'll want to ramp up gradually to your target distance. Personally, I use a barebones Garmin. I prefer to keep my phone in my pocket. Saves the battery, safer in a fall, etc. But obviously lots of riders use them, that's why they sell the phone mounts. And even without a device, you can just pick a destination, check the distance on a map or mapping app, and ride there.