After you have cleared the game for the first time from Day 4 to Day 160. You will unlocked Realistic Mode which is far more longer than the normal days you have been playing.

Since it is named "Realistic", I guess the developer is giving the player a chance to feel what it likes to be left on read in real time where some people will take hours and hours before reading your message or before replying to you, therefore Realistic Mode took a lot of hours to even get a "read" or "reply" from the message in the game.

In addition, there have been some reddit post forum discussing on how to complete Realistic Mode and some people said to have waited 3-12 hours per reply in Realistic Mode and took a total of 60 hours complete the whole Realistic Mode.

However, there is a much faster way to complete Realistic Mode with the use of Cheat Engine. The latest Cheat Engine version is 7.2 which releases for Windows and Mac users on November 1st, 2020 and can be found here: https://www.cheatengine.org/

A website created by CTcommander Senpai, also recommends players to use Cheat Engine to complete the Realistic Mode much faster. You can check it here: https://gameplay.tips/guides/7711-left-on-read.html

Using Cheat Engine on "Left on Read (Realistic Mode)" enables player to obtain speedhack and alter the speed. It can speed up the message incredibly fast than normal. The Cheat Engine provides increases speed up to 500 times faster than normal.

In order to use the Cheat Engine, you need open the Left on Read game then open the Cheat Engine. Next, you need to click the little icon underneath "File", then you find the LeftOnRead.exe and click that. A lot of stuff will appear on the right side and you'll be able to find the "Enable Speedhack" checkbox, by clicking that you'll be able to alter the speed. (For this Realistic Mode, it is best to apply the maximum speed given in the Cheat Engine which is 500 to get the fastest time.)

In order to speedrun (Realistic Mode), it is best for every speedrunners to use Livesplit because its easier and more accurate timings. In Realistic Mode, there are a total of four texts with the last text being "/hpbpcsyprz". The speedrunner will start their time when screen moves after pressing the Play button, and proceeds to split everytime you received a reply and ends the time when you received or jump on the last text.

After the last text of "/hpbpcsyprz" message appears, you have successfully completed Realistic Mode and you unlocked the Realist Achievement.

In addition, I believe the most optimized time for Realistic Mode will be 35 minute or 34 minute.