How far should I sum?
APL (Dyalog), 13 bytes

-1 bytes thanks to ngn



{⊃⍸⍵≤+\÷⍳⌈*⍵}


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A dfn solution that takes a right argument.

Explanation:

{           }   ⍝ dfn
 ⊃              ⍝ Take the first of
  ⍸             ⍝ The indexes of the truthy values of
   ⍵≤           ⍝ The right argument is smaller than or equal to
     \+         ⍝ The cumulative sum
       ÷        ⍝ The reciprocal of each of
        ⍳       ⍝ The range 1 to
         ⌈      ⍝ The ceiling of
          *⍵    ⍝ e to the power of the right argument