For the purposes of personal independence payment (PIP), you should score points for the descriptor that applies to you for the majority of the time, though this will depend to a large extent on the opinion of the medical assessor.
Guidance issued by the DWP says that your ability should be measured over the three months prior to your claim and also over the nine months following your claim. So, if you satisfy a descriptor for at least 50% of the time in both of these periods, then you should be awarded pints for it.
Where your condition fluctuates so much that you could meet several different descriptors over the course of a year, you will score points for the one that would apply for the greatest proportion of the time.
