What happens when a customer sends STOP?
They are opted out at once and left out of every campaign until they send START.
If a customer sends STOP (or unsubscribe, stopall, optout, opt-out), they are opted out straight away and get a short reply confirming it. They will not be included in any campaign.
If they send START (or unstop, subscribe, resubscribe), they are opted back in.
Only the whole message counts
The word must be the whole message. "Please don't stop" does not opt anyone out. Capital letters don't matter.
Change the words and replies
In Settings → Opt-in and opt-out you can add your own words in any language (Hindi too) (1) and write your own reply (2). You can also stop messages from your own software reaching people who opted out.
See who opted out
Reports → Download → Opt-ins and opt-outs gives a list with dates.