The timeline also depends on which Office you will be going to. SAMUI has new rules now.
1. 6 weeks after (your 90 day) entry, you visit the office with your form filled out and all supporting documents EXCEPT the bank ones and your Medical Certificate. At that time, they book you in for an appointment to return closer to the date with the remaining documents.
2. You turn up at the appointment time with your documents. They will eventually get partially processed, you pay the fee, get a ticket number and you are asked to come back in a week’s time to collect your passport.
3. You go back in a week with your ticket and you get your passport back with your extension stamped.
Please note that this applies to SAMUI. I would check with your local Immigration Office as soon as you arrive.