There might be a chance to get this - but it will involve some effort.
The Cisco ASA VPN sensor indeed shows you the amount of users connected - in theory that might be a single SNMP query and single response from the ASA (did not look it up in detail).
What you want is the duration of a sessions - and now it becomes complicated.
You might have 40 users connected to your ASA - so do you now want to report the top time you are able to find or what exactly?
In theory let us say you have a license for 500 users - PRTG allows up to 50 channels per sensor - well - the time alone won't help so you need additional values ... since there are IPs involved that you could only report as a text-string what narrows it down to 1 per sensor but in theory abusing the warning and error fields per channel you might be able to bypass this...
So - assume we set the error message in the channel as the IP found in the ASA - so if the session time is more then 24 hours you get the IP as an error message - you still end up with 100 sensor a 50 channels - just to cover all possible VPN connections.
Why? Simply because if you look at this MIB file you will quickly find that a big part will be in the OID 22.214.171.124.126.96.36.199.392.1.3 - possibly even the session time hidden somewhere (likely as a tick-count (Linux/Unix) or a seconds value) but you will have changing and extremely variable OID paths due to the way it works. You still would need to cover all possible connections and the results back to PRTG would highly vary.
On top - you would need to find a way to do the SNMP queries - likely in PowerShell and probably with this free module (as of what I read it is free):
Cause you will need to act dynamically on it.
Even just to find out the longest session - you will need to include a script technique to read through the current connections and determine the oldest session/connection - assuming that Cisco provides you that per SNMP (likely - but did not test it out).
All of this is quite a bit of effort - it sure is possible - but having a ASA here myself I still wonder what good it would do me to know this. Theoretically I could abuse SYSLOG for it or the logging and later determine such information - if not directly in clear-text reported - simply due to database queries looking for connect and disconnect information - assuming your logging is reported to a database system. We talk about big-data here - while PRTG can process and help you here - it is not a database where you can do syslog research.
If you are worried about the time the users are connected - there are a few configuration values on the ASA that might help to prevent this.. though, I have users that are connected many days - due to them having a home office with of course company provided equipment (nothing else is allowed to use the VPN) where I really don't care if they are connected for a whole month - it actually allows some other automatic stuff to run on their systems - likely pushed out software or various other scheduled maintenance tasks.
Sorry for the long response and not providing you a solution - I don't see any benefit in putting so much effort in it - but I know it might be possible if you dig a bit deeper in to the provided links.