check time from logged in to now

Hello,

I want to check user's log in time with he/she logged in to Now(). it seems not working for me. here is my code

$result            = $op->runsql("SELECT TIMESTAMPDIFF(MINUTE, a.last_access, NOW()) AS MinsSinceLastLogin FROM ".tbl_author." a WHERE a.id = '$blogdata->author'");

if(mysql_num_rows($result)>0){

while($row = $op->select($result)){

$minutes_dif = $row1['MinsSinceLastLogin'];

$last= NOW() - $minute_dif;

if($last <600){ // considering here 10 min, but not sure about it
echo "online";
}else{
echo "offline";
}

}}

and the second question is how many minutes equals to 20 minutes? brcause when i tried this query in to sql. have got this

MinsSinceLastLogin

717

thanks in advance.