Hey!
If you want to stop reading at certain points, you can use the <pause> tag, and then call Continue() to keep reading from there. You can also use the timing tag <t=5.0> to make the following letters start reading from a specific time.
If you mean length as in time, you should be able to use... stm.info[stm.latestNumber].readTime. That should give you the time the latest character was printed out at.
I hope this works for you! Let me know if I misinterpreted what you meant!