You don't gain any playtime from AFKing, if your character is stationary for longer than 5 minutes then your playtime counter will stop and only resume once your character moves.You can afk all the hours anyways.
On-topic: Throughout the many years of playing RS private servers, the veteran rank has always been something that should be somewhat of a challenge to acquire. And I do have to agree, having 50 days in-game playtime as a requirement/prerequisite is inconsiderably too high. To counter your suggestion, in my books it would be more ideal if the administration was to decrease the in-game playtime requirement to ideally 45 days in-game, or even 40. Minusing 5/10 days off the requirement still makes it difficult to accomplish the milestone needed but makes it just that little bit extra easier to attain.
I'm intrigued to see what feedback this thread receives.