Question about auto-save

[jbv at souslelogo.com]

Hi list,
I am working on a web app (with LC server-side) in which end users
will be able to add & manipulate various svg elements to complete
custom designs.
My client asked me to implement some kind of auto-save function so
that users can automatically retrieve the last step of their design
each time they log on to the site. I am planing to use cgi requests
to record the svg tags & attributes on the server.
I was wondering : what is the best way to handle this :
- send a cgi request every time the user changes something
- send a cgi request every minute (for instance) and record the
content of the design at that specific time
- send a cgi request only when users leave the page or the site,
and record only the last step of the design during a specific session...

Thanks in advance for any useful tip & advice.
Best,
jbv