Get the length of a Sumac Sequence
x86 machine code (8086), 15 13 bytes
-2 bytes thanks to @EasyasPi.
00000000  31 C9 48 7C 07 40 41 29-D8 93 EB F6 C3            1.H|.@A).....

Callable function.
Expects AX = t1, BX = t2. Output is to CX.
Disassembly:
31C9          XOR     CX,CX    ; Set CX to 0
          LOP:
48            DEC     AX       ; --AX
7C07          JL      END      ; If AX &lt; 0, jump to 'END'
40            INC     AX       ; ++AX
41            INC     CX       ; ++CX
29D8          SUB     AX,BX    ; AX -= BX
93            XCHG    BX,AX    ; Swap AX and BX
EBF6          JMP     LOP      ; Jump to tag 'LOP'
          END:
C3            RET              ; Return to caller

Example run
Tested with DOS debug:
-r
AX=0078  BX=0047  CX=0000  DX=0000  SP=FFEE  BP=0000  SI=0000  DI=0000
DS=0B17  ES=0B17  SS=0B17  CS=1000  IP=0000   NV UP EI PL NZ NA PO NC
1000:0000 31C9          XOR     CX,CX
-t

AX=0078  BX=0047  CX=0000  DX=0000  SP=FFEE  BP=0000  SI=0000  DI=0000
DS=0B17  ES=0B17  SS=0B17  CS=1000  IP=0002   NV UP EI PL ZR NA PE NC
1000:0002 48            DEC     AX
...
-t

AX=FFFA  BX=0020  CX=0005  DX=0000  SP=FFEE  BP=0000  SI=0000  DI=0000
DS=0B17  ES=0B17  SS=0B17  CS=1000  IP=000C   NV UP EI NG NZ NA PE CY
1000:000C C3            RET
```