Always
Because the slant length is the longest length when looking at the Pythagoras theorm
In this case the slant is y, with x and y as the height and base
y²=

Hence y the slant is the sum of the height and and base and will always be larger
A better estimate would be something that is closer to one you had before...
14^ 0.08t = 56.
From here, you can rewrite it as log base 14 argument 56 = 0.08t.
When you plug it in t is about 19
Answer: ≈19 years