Hi irfanali,
Making members 24 hours online in a phpbb forum need advanced hacks. So I never recommend it. But you can easily do a hack so that you can make your phpbb forum members online for a maximum of 999 minutes.
First thing you need to do is change the active users from 5 minutes to 600 minutes from the ACP of your phpbb forum.
To do this Go to your Admin Control Panel of phpbb >> Under General settings, you can see
Load Settings link>> click
Load Settings >> View online time span [Number of minutes after which inactive users will not appear in “Who is online” listings. The higher this value the greater is the processing required to generate the listing.] >> Now change this to 600 minutes if you wan to make users online for 10 hours. Here the maximum allowed value is "999"
In the screen shot given below, you can see it

Now you need to Change the display duration of active users
OPEN:
Code:
language/en/common.php
FIND:
Code:
'VIEW_ONLINE_TIMES' => 'based on users active over the past %d minutes',
REPLACE WITH:
Code:
'VIEW_ONLINE_TIMES' => 'based on users active over the past 5 minutes',
SAVE IT.
Now go to the General tab of your phpbb ACP >> Purge the cache by clicking on RUN NOW button. Check the home page and enjoy.