Uptime is the term used to define the duration a machine or electronic device is in operation. Here are the steps to check for how many duration the mobile was ON continuously.
Step 1 : Navigate to "About phone" in your device settings.
Step 2 : Navigate to "Status" in About phone.
Here you can find Uptime. In the below screenshot, its displayed as mobile is ON for 27 plus hours. Uptime will get reset after every switch off of your mobile.