You get it back in your OTP bank. The date you must use it by doesn't change. Going off of memory here because I don't feel like looking it up, but I believe it's the second month after you get it. Take a look at your crew information screen and it will tell you what month it expires.
People thought recycling OTP might have been a technique used by the Wolfpack to get charters. The thought was that one could pick up a charter using OTP/PMU, wait until 4 AM, drop the trip into open time, then immediately pick it back up at general makeup. Nothing in the CBA prevents that, but if that was a technique it would have been nice to fix that in the TA. There was also some thought there was a bug in the system that someone had discovered that would allow one to not only recycle OTP, but to keep extending the "use by" date.
I don't think anyone has a problem with someone not wanting a trip picked up with PMU, dropping it, then using the PMU on another trip later. Remember when you elected OTP, you retained the pay and you will see a deduct for that pay the second month, so don't be surprised by that.