[Konstantinos]

pawn Код:

// global:
new ConnectTime[MAX_PLAYERS];

// OnPlayerConnect:
ConnectTime[playerid] = gettime();

// command /onlinetime:
...
format(string, sizeof(string), "[NOTE] - You've been online for %d minutes!", gettime() - ConnectTime[playerid]);
...

It will show you how many seconds you've been online. There are many posts/threads about converting to hours/minutes etc, search.