I‘m a new Linux system user. How do I check how long a process or pid has been running on my Ubuntu Linux server?

You need to use the ps command to see information about a selection of the active processes. The ps command provide following two formatting options:

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1. etime Display elapsed time since the process was started, in the form [[DD-]hh:]mm:ss.
2. etimes Display elapsed time since the process was started, in seconds.

How to check how long a process has been running?

You need to pass the -o etimes or -o etime to the ps command. The syntax is:
ps -p {PID-HERE} -o etime
ps -p {PID-HERE} -o etimes

Step 1: Find PID of a process (say openvpn)

$ pidof openvpn
6176

Step 2: How long a openvpn process has been running?

$ ps -p 6176 -o etime
OR
$ ps -p 6176 -o etimes
To hide header:
$ ps -p 6176 -o etime=
$ ps -p 6176 -o etimes=
Sample outputs:

The 6176 is the PID of the process you want to check. In this case I’m looking into openvpn process. Feel free to replace openvpn and PID # 6176 as per your own requirements. In this find example, I am printing PID, command, elapsed time, user ID, and group ID:
$ ps -p 6176 -o pid,cmd,etime,uid,gid
Sample outputs:

  PID CMD                             ELAPSED   UID   GID
 6176 /usr/sbin/openvpn --daemon        15:25 65534 65534