Altitude Acclimatisation is process that results in the body adapting to lower levels of oxygen at altitude by producing more red blood cells.
It is achieved by spending extended periods of time at various levels of altitude before progressing higher, it can also be achieved using an altitude tent.
Altitude Acclimatisation is important if you are taking place in an event held at an elevation substantially higher than that at which you usually train.
General wisdom suggests that two weeks is a reasonable length of time to gain a noticeable boost in your red blood cell count. Less than that and you probably won’t get a very noticeable or worthwhile boost.
You also have a longer-term window, another couple of weeks, during which the unwinding is happening, though your blood cell count is still enhanced. After that, the benefits are likely to have disappeared.