I think chastity should have a hard cap of 3 days. To me, it seems a little overpowered when you can keep getting it on you for 6 or 7 days at a time. Of course, if the other kingdom is active enough, they can still keep it on you constantly, but it seems way too easy at the moment, especially since it's not a difficult spell to successfully cast. Having a situation where a couple of mystics in the enemy kingdom can inflict zero birthrates on almost your entire kingdom constantly doesn't seem very balanced. In my opinion, since chastity severely limits any form of peasant recovery, it should have a very low maximum duration. Either that, or it should be made a lot harder to cast.
Otherwise, please create a spell that gives a certain chance of curing chastity each tick. 'Promiscuity' perhaps?