A chart explaining the hair growth rate Full beard trimming.
To add to the game's realistic atmosphere, the hair and facial hair of Arthur Morgan and John Marston slowly grow as they explore the game world. This progress can be hastened by consuming Hair Tonics, and beards can then be trimmed to specific styles at any Barber Shop or at a shaving kit found at the Van der Linde gang camp and eventually Beecher's Hope. Hair can only be cut at barbershops.
Red Dead Online
The player character's hair and/or facial hair doesn't grow in regular gameplay, and instead they can purchase instant styles from any barbershop.
While the hair growth mechanic is relatively glitch-free, there are several reports of problems with beard growth; While some players report they can grow a stage 10 beard as early as Chapter 2, others report that they cannot grow one even in the epilogue no matter how many hair tonics they consume. It has also been reported that some parts of the facial hair grow faster than others, most often the mustache.
Goatee shapes and styles
Sometimes Arthur or John's beard is gone upon loading a save game. Changing clothes seems to fix this.
Ever since the day one patch, beard and hair growth has been broken for the vast majority of players. The beard behaves as stated above and the hair ceases to grow at an accelerated rate.
The hair growth doesn't happen in real time, and as such they player will need to occasionally trigger a loading screen (such as fast-traveling or sleeping) to allow the hair to progress to the next stage.
Arthur's hair grows naturally right parted from a bald haircut whearas John's hair grows left parted.
A quick way to grow hair and beard is by purchasing a large amount of hair tonics, going to a remote or lawless location (Wallace Station, Armadillo or Van Horn) and murdering a shopkeeper. After this walk outside and the doors to the shop will lock, and the prompt to "Wait until morning" prompt appears. Unless the player leaves the area, this prompt will appear every time the player is near the door, allowing the player to skip a large amount of days quickly; simply take a few tonics every few in-game days and keep skipping to the next morning. From level 7 onwards this will require several weeks worth of skips, however.