While using end-to-end encryption may be enough security for most of your messages in Telegram, you can go a step beyond this by sending messages that disappear after an allotted amount of time. This prevents you from having to manually delete the message at a later time, so you can send it and forget it without worrying about whether or not it's still out there.
To start a self-destructing message in Telegram, you'll have to be in a "Secret Chat." You cannot send a self-destructing message in a regular chat, so make sure it's a Secret Chat you're in. If not, start one.
If you're using an iPhone, tap on the stopwatch icon next to the message field area when in a Secret Chat. On an Android phone, either tap on the ellipsis (three dots) in the top right and select "Set self-destruct timer" or just tap on the little stopwatch icon by the person's name or number up top.
From there, you have multiple lengths of time to choose from — 1 to 15 seconds, 30 seconds, 1 minute, 1 hour, 1 day, or 1 week. Whatever you choose, the timer won't kick in until the message is being viewed on the recipient's screen (you'll see two green check marks next to the message when it starts counting down). But what message, right?
Any message you send to your recipient after setting a timer, or any message they send to you afterward, will have the same time restraints for viewing. It does not apply to just the one message you plan on sending — it applies to all new messages in the Secret Chat. You can't just set a timer for one message and that's it.
Well, technically you can send just one timed message — by setting a self-destruct timer, sending your message, then disabling the timer in the Secret Chat. Just note that the timer does not affect messages sent or received before it was set, nor after it is disabled.
If you decide to send a photo while the self-destructing timer is enabled in the Secret Chat, it will be hidden initially, and the timer won't start until the recipient taps on it to open it. And unlike with written messages, photo messages will show both you and the recipient a countdown clock.
For videos, the recipient must first download the video, then tap to open it. Plus, if the video is longer than the timer, it will be given a new timer the length of the video, so they can at least watch the whole thing.