I would just estimate if you think you go on The Beast a little more, a little less, or about the same as Banshee. Based on that adjust your 116 figure up or down a little, then multiply that number by 3 to adjust from 7 years to 21 years.
For example, you think you go on Beast 90% as often as Banshee, 116 x 90% = 104. Then 104 x 3 = 312 for your estimate.