I change them after every smoke. Heck, even during mid-smoke: either the Dr Perl's filters sux or I got a bad batch, because they immediately start imparting a plasticky flavour to the smoke that totally annihilates all flavours from the tobacco.
Too bad I got a box of 180 of the things: it's practically full.
I've been smoking my Vauen Basic with an adapter due to this problem, and not smoking my Quixx at all because the adapter is too long for the latter. I've been wanting to try balsa but @ $40 USD (~$53 CAD) plus shipping for a box of 200 I've given them a miss heretofore.