Slayer XP charges do vary according to the task but players that usually go for efficiency as an alternative to tending to experience a set of monsters that they can rotate through via the application of slayer points and/or slayer VIP passes.

Going 99 for you to 120 Slayer at the moment takes around 350 hours which using the average of 260k/hr. While levels 1 to 99 does take around 150-200, determined by combat levels, Runescape 2007 Gold and reward/quest open progress.

This can be something Jagex would like to achieve with Slayer staying raised to 120. Bearing that in mind, they would count on 99 to 120 Slayer to look at around 150 a long time and average out and about to around 600k XP/hour ultimately.