Looks like validity is extended by recharging only. So far, placing a call and sending an SMS haven't changed the expiration date, even though the remaining credit changes on the Bouygues website.
http://www.assistance.bouyguesteleco...carte-prepayee
Votre ligne reste ouverte 12 mois Ã* compter de votre dernier rechargement. Vous êtes toujours joignable!
Another thing, the phone was in a drawer for a few weeks, and the first time I switched it on after it had become a Bouygues account it took a while before it was able to do anything (send/receive calls, send messages). I was afraid it had expired, but then after 15 minutes, everything was fine.