How can I log the start and stop connection with known WI-FI networks
I will answer my own question until someone else shows me another application.

I have found the following one in the market : Wifi Monitor.

Time will tell whether it really does what it promises.

Still, if anyone has got some other software which does this, please let me know !

UPDATE

After trying this app for a week now, I can confirm that it works. When I enter/leave buildings with a known WiFi connection, the start and end of the connections are logged. This gives me an easy although crude way of hour-logging.