A newbie asks...

[Kobal2]

Yeah, the equation is fairly simple : if after the battle, you still own the province, PD is immediately back to full. This happens even if that province takes multiple successive attacks BTW - PD will replenish in between each fight.

If at the end of the battle, someone else owns the province, PD is set back to 0 for everyone - the loser won't get back his PD by retaking the province, either. So if for instance you teleport a guy in, wipe the PD, then your thug gets bumped off by an invading army moving in the province in the same turn, your opponent will have kept his province but lost all the PD.
Which is why few people invest too much into it generally, since a decent raiding party can cost you all that sunk investment.

General rules*: 1 sees off non-giant scout takeovers, 5 takes care of single casts of call of the wind / arouse hunger / call of the wild, 10 will catch a scout sneaking around once a decade and 15 will usually resist random barbarian attacks and the like which can be handy if you've got high misfortune scales.

Anything above that is probably a waste for general purposes, although dropping some quick PD just before a major defensive battle can be sensible if you have spare change and the additional soldiers won't be a liability.

*Disclaimer: these numbers are general values provided for information purposes only and the exact thresholds vary in practice depending on the soldiers your PD gives you. Your mileage may vary. By purchasing this PD you implicitly accept that it will suck, as well as any and all consequences of such suckage. Do not feed your PD after midnight. Warranty void if removed. If your PD lasts for more than 4 hours, consult a physician.