What does it mean when the number (minutes) has disappeared from besides someone's phone icon in chat?

[Tequilasaurus]

I know the number is the minutes or hours or whatever since they were last active, but I was active 30m ago and now on my friend's chat all of a sudden it's not showing a number now?

I'm still logged into fb chat...what happened? All other phone icons still show a number

[Sky..]

It won't show any number, because you are already active. Hence it will show 'active now' by the side of mobile icon. It will show number when you log out from your account on you friend's chat list.