Length of a Sumac sequence
Husk, 8 bytes

→V&lt;¡oG-↔


Takes input as a 2-element list.
Try it online!

Explanation

→V&lt;¡oG-↔  Implicit input, say p=[101,42]
   ¡      Iterate on p:
       ↔    Reverse: [42,101]
    oG-     Cumulative reduce by subtraction: [42,59]
          Result is infinite list [[101,42],[42,59],[59,-17],[-17,76],[76,-93]...
 V&lt;       Find the first index where adjacent pairs are lexicographically increasing.
          In our example [42,59] &lt; [59,-17], so this gives 2.
→         Increment: 3