For me ... it was 800 km or 3 weeks of cycling. Mine suddenly broke in 400 km into a 1000 km randonnee.

As for proofide, I put on a thin layer all over my saddle when I got it ... then again at about the 6 month mark ... then again about a year later. You don't have to go crazy with proofide or other things some may recommend. A teensy dab of proofide will do. I bought my saddle in 2004, and my container of proofide hardly looks like it has been used.