Vitamin B12 deficiency is pretty rare even considering the fact that you cannot get B12 from eating a strict plant-based vegan diet.
Because your body is able to store B12 (in your liver and kidneys), it will take some time of going without any B12 before you notice any signs and symptoms that you are deficient.
Some people notice symptoms within 3 to 5 years and other people notice them in a year. Some people never notice them at all. Everyone's body is different.
Not getting enough B12 could also put you at an increased risk for heart disease and stroke.
If you have symptoms of deficiency, those symptoms can be reversed by taking a B12 supplement.
If you allow the symptoms to go on for too long though, they will get worse. You could even end up with permanent nerve damage.
If you suspect that you may have a deficiency, speak to your health care practitioner about it. Some of these same symptoms can be caused by other health problems, so it's important to rule those out.