Having worked for a major phone carrier in tech support for the last 3 years I have noticed that a lot of the common procedures for androids typically recommend to have at least 150 MB of storage available to run efficiently with your basic preinstalled apps.
But I have gathered that the more apps you download on your phone, the more notifications and updates are constantly being pushed to your phone in the background which will slow it down quite a bit the lower the available amount of space gets.
Typically disabling automatic push updates, notifications and turning the data off in the background will help to keep your battery life longer and your phone running smoothly as long as you remember to keep going in and manually updating your apps.
If you use multiple social media apps and streaming video apps take lots of photos and videos, text and call a lot as I personally do, while continuing to keep updates and notifications on, I would personally recommend having at least 500 MB to 1 GB free. And make sure to have photos and videos synced up to a cloud service like google photos or dropbox so you can free up space on your phone quickly.
Some people use SD cards which can help, but I have noticed SD cards can be easily corrupted and if not purchased in the right speed class or storage size can actually slow your phone down.