The length (amount of data to be pushed into the stack) typically comes directly after the OP_ code.
If the byte after OP_RETURN (
0x6A) is between
0x4B, that is signaling the amount of data to push into the stack. This is defined in the Script language; a chart can be found here.
The first byte
0x6A decodes to
OP_RETURN and the next byte is
to push 36 bytes of data to the stack. (
0x24 hexadecimal is 36 in decimal)
Our example therefore decodes to:
OP_RETURN PUSHDATA(36) AA21A9EDA45B20BAE4C0D9C747D7C17074D0606AF70FABA1394FC75E5DA6ABDA0426232E
If there was any more data after the 36 bytes it would be interpreted in coordination with the proper OP_code.