Men typically start developing facial hair in the later stages of puberty or adolescence, full beard growth is possible starting at around age 18. https://en.wikipedia.org/wiki/Facial_hair.
So, how long does it take to grow a full beard? It depends on many factors, like age, ethnic, hormones and also genetics  https://pubmed.ncbi.nlm.nih.gov/5173315/.
According to study https://onlinelibrary.wiley.com/doi/full/10.1111/ics.12328, beard hair grows fast. The average daily growth rate of beard hair ranges between 0.3 and 0.5 mm, which is similar to the growth rate of scalp hairs and is about twice as fast as that of eyebrow hairs. The rate of hair growth also depends on additional environmental and endogenous factors such as hair pigmentation. For example, white beard hair reportedly grows more than twice as fast as pigmented hair.
How long does it take to grow a full beard? For most men it takes an average two to six months.
Here are some really useful tips on how you can grow a thicker beard faster https://www.mobros.co.uk/blogs/blog/how-to-grow-a-thicker-beard-faster.
1. Go to bed early: If you can’t go to bed early, sleep in late. The level of growth hormone in the blood increases when you sleep, speeding up cell reproduction and hair growth in the process. This peaks around 2am, so aim to be asleep well before then.
2. Exercise: The more sleep you get, the more energy you’ll have to exercise. Regular exercise increases testosterone, which promotes hair growth. It also boosts blood circulation, giving the hair the nutrients it needs to grow.
3. Eat healthy: Combine exercise with a healthy diet loaded with fruit, vegetables, nuts and protein. Fruit and veg give your body the vitamins and minerals it needs, while protein keeps the beard looking healthy.
4. Drink enough water daily: This is healthy living advice 101 but getting in your eight glasses of water a day will flush out the harmful toxins cramping your beard’s style. Hydrated skin is healthy skin and healthy skin means a lusciously thick beard.
5. Quit smoking: Smoking is also bad for beards. They should put that on the side of a cigarette packet.