[Sascha]

Код:

new last[MAX_PLAYERS];
COMMAND:hello(playerid, params[])
{
    new string[256];
    if(GetTickCount() - last[playerid] < 5000)
    {
        new towait = floatround((5000 - (GetTickCount() - last[playerid])) / 1000, floatround_ceil);
        format(string, sizeof(string), "You have to wait %d seconds", towait);
    }
    else
    {
        last[playerid] = GetTickCount();
        //your hello command code
    }
    return 1
}

The keyword would be "GetTickCount" for you https://sampwiki.blast.hk/wiki/GetTickCount
not tried to compiled, so typos or missing brackets might be possible...